Method for transmitting or receiving frame in wireless LAN system and apparatus therefor

ABSTRACT

According to an embodiment of the present invention, a method for transmitting a frame by a station (STA) in a wireless LAN system supporting a high efficiency physical layer protocol data unit (HE PPDU) comprises the steps of: configuring a first duration field included in an HE-SIG A field; and transmitting a frame including the HE-SIG A field and an MAC header, wherein the first duration field may be configured to indicate a transmission opportunity (TXOP) value using a number of bits smaller than that of a second duration field included in the MAC header, and a granularity of a time unit used in the first duration field in order to indicate the TXOP value may be configured to be different from a granularity of a time unit used in the second duration field.

This application is a continuation of U.S. patent application Ser. No.15/520,822, filed on Apr. 20, 2017, currently pending, which is theNational Stage filing under 35 U.S.C. 371 of International ApplicationNo. PCT/KR2016/005097, filed on May 13, 2016, which claims the benefitof U.S. Provisional Application No. 62/160,614, filed on May 13, 2015,62/163,984, filed on May 20, 2015, 62/259,078, filed on Nov. 24, 2015,62/276,246, filed on Jan. 8, 2016, 62/294,310, filed on Feb. 12, 2016,62/297,938, filed on Feb. 21, 2016, 62/302,202, filed on Mar. 2, 2016,and 62/304,304, filed on Mar. 6, 2016, the contents of which are allhereby incorporated by reference herein in their entirety.

TECHNICAL FIELD

The present invention relates to a method of transmitting or receivingframes in a wireless LAN system and, more particularly, to a method oftransmitting and receiving frames for management of a transmissionopportunity (TXOP) or network allocation vector (NAV) and an apparatustherefor.

BACKGROUND ART

Standards for Wireless Local Area Network (WLAN) technology have beendeveloped as Institute of Electrical and Electronics Engineers (IEEE)802.11 standards. IEEE 802.11a and b use an unlicensed band at 2.4 GHzor 5 GHz. IEEE 802.11b provides a transmission rate of 11 Mbps and IEEE802.11a provides a transmission rate of 54 Mbps. IEEE 802.11g provides atransmission rate of 54 Mbps by applying Orthogonal Frequency DivisionMultiplexing (OFDM) at 2.4 GHz. IEEE 802.11n provides a transmissionrate of 300 Mbps for four spatial streams by applying Multiple InputMultiple Output (MIMO)-OFDM. IEEE 802.11n supports a channel bandwidthof up to 40 MHz and, in this case, provides a transmission rate of 600Mbps.

The above-described WLAN standards have evolved into IEEE 802.11ac thatuses a bandwidth of up to 160 MHz and supports a transmission rate of upto 1 Gbits/s for 8 spatial streams and IEEE 802.11ax standards are underdiscussion.

DISCLOSURE Technical Problem

An object of the present invention devised to solve the problem lies ina method of efficiently signaling a TXOP duration by a TXOPholder/responder STA through frame transmission in a wireless LAN systemsupporting an HE PPDU and a method of accurately managing a NAV by athird party STA that receives signaling of the TXOP duration through acorresponding frame.

The present invention is not limited to the above technical problems andother technical objects may be inferred from embodiments of the presentinvention.

Technical Solution

In an aspect of the present invention, a method of transmitting a frameby a station (STA) in a wireless LAN system supporting an HE PPDU (highefficiency physical layer protocol data unit) includes: setting a firstduration field included in an HE-SIG A field; and transmitting a frameincluding the HE-SIG A field and a MAC header, wherein in setting of thefirst duration field included in the HE-SIG A field, the first durationfield is set to indicate a TXOP (transmission opportunity) value using asmaller number of bits than a second duration field included in the MACheader, and wherein a granularity of a time unit used for indicating theTXOP value in the first duration field is set to be different from agranularity of a time unit used in the second duration field.

In another aspect of the present invention, a station transmitting aframe in a wireless LAN system supporting an HE PPDU includes: aprocessor for setting a first duration field included in an HE-SIG Afield; and a transmitter for transmitting a frame including the HE-SIG Afield and a MAC header, wherein in setting of the first duration fieldincluded in the HE-SIG A field, the first duration field is set toindicate a TXOP value using a smaller number of bits than a secondduration field included in the MAC header, and wherein a granularity ofa time unit used for indicating the TXOP value in the first durationfield is set to be different from a granularity of a time unit used inthe second duration field.

In another aspect of the present invention, a method of managing anetwork allocation vector (NAV) by a station (STA) in a wireless LANsystem supporting an HE PPDU includes: receiving a frame including anHE-SIG A field and a MAC header; and performing NAV management based onone of a first duration field included in the HE-SIG A field and asecond duration field included in the MAC header, wherein the firstduration field is set to indicate a TXOP value using a smaller number ofbits than the second duration field included in the MAC header, andwherein a granularity of a time unit used for indicating the TXOP valuein the first duration field is set to be different from a granularity ofa time unit used in the second duration field.

The granularity of the time unit used in the first duration field mayvary depending on the TXOP value to be indicated through the firstduration field.

The first duration field may include at least one bit indicating thegranularity determined according to the TXOP value. The remaining bitsof the first duration field may indicate how many number of time unitsbased on the indicated granularity are included in the TXOP value.

The first duration field may be set to 5, 6 or 7 bits and the mostsignificant bit (MSB) of the first duration field may be used toindicate the granularity of a time unit. The first duration field may beset to 5 bits and the granularity indicated by the MSB may be one of 32μs and 512 μs, the first duration field may be set to 6 bits and thegranularity indicated by the MSB may be one of 16 μs and 256 μs, or thefirst duration field may be set to 7 bits and the granularity indicatedby the MSB may be one of 8 μs and 128 μs.

Both the TXOP value indicated by the first duration field and a TXOPvalue indicated by the second duration field may be set for transmissionof the same frame, and the TXOP value indicated by the first durationfield may greater than or equals to the TXOP value indicated by thesecond duration field.

The STA performing NAV management may set, update or reset a time wherechannel access is restricted in order to protect a TXOP of a transmitterof the frame or a receiver of the frame when the STA is not designatedas the receiver of the frame.

The STA performing NAV management may perform NAV management on thebasis of the second duration field when the MAC header has beensuccessfully decoded and perform NAV management on the basis of thefirst duration field when decoding of the MAC header has been failed.

Advantageous Effects

According to an embodiment of the present invention, a TXOP duration isset in an HE-SIG A field and thus even third party STAs that do notdecode a MAC header can accurately protect a TXOP of a TXOPholder/responder. Furthermore, it is possible to minimize signalingoverhead of the HE-SIG A field by using multiple granularities of timeunits for a TXOP duration field set in the HE-SIG A field.

Other technical effects in addition to the above-described effects maybe inferred from embodiments of the present invention.

DESCRIPTION OF DRAWINGS

FIG. 1 illustrates an example of a configuration of a wireless LANsystem.

FIG. 2 illustrates another example of a configuration of a wireless LANsystem.

FIG. 3 illustrates a general link setup procedure.

FIG. 4 illustrates a backoff procedure.

FIG. 5 is an explanatory diagram of a hidden node and an exposed node.

FIG. 6 is an explanatory diagram of RTS and CTS.

FIGS. 7 to 9 are explanatory diagrams of operation of an STA that hasreceived TIM.

FIG. 10 is an explanatory diagram of an exemplary frame structure usedin an IEEE 802.11 system.

FIG. 11 illustrates a contention free (CF)-END frame.

FIG. 12 illustrates an example of an HE PPDU.

FIG. 13 illustrates another example of the HE PPDU.

FIG. 14 illustrates another example of the HE PPDU.

FIG. 15 illustrates another example of the HE PPDU.

FIG. 16 illustrates another example of the HE PPDU.

FIGS. 17 and 18 illustrating an HE-SIG B padding method.

FIG. 19 is an explanatory diagram of uplink multi-user transmissionaccording to an embodiment of the present invention.

FIG. 20 illustrates a trigger frame format according to an embodiment ofthe present invention.

FIG. 21 illustrates an example of NAV setting.

FIG. 22 illustrates an example of TXOP truncation.

FIG. 23 illustrates TXOP duration setting of multiple granularitiesaccording to an embodiment of the present invention.

FIG. 24 illustrates allocation of a UL OFDMA BA frame in MCS0 accordingto an embodiment of the present invention.

FIG. 25 illustrates a method of setting a TXOP duration value accordingto an embodiment of the present invention.

FIG. 26 illustrates a frame transmission and NAV management methodaccording to an embodiment of the present invention.

FIG. 27 illustrates an apparatus according to an embodiment of thepresent invention.

MODE FOR INVENTION

Reference will now be made in detail to the exemplary embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. The detailed description, which will be given below withreference to the accompanying drawings, is intended to explain exemplaryembodiments of the present invention, rather than to show the onlyembodiments that can be implemented according to the present invention.

The following detailed description includes specific details in order toprovide a thorough understanding of the present invention. However, itwill be apparent to those skilled in the art that the present inventionmay be practiced without such specific details. In some instances, knownstructures and devices are omitted or are shown in block diagram form,focusing on important features of the structures and devices, so as notto obscure the concept of the present invention.

As described before, the following description is given of a method andapparatus for increasing a spatial reuse rate in a Wireless Local AreaNetwork (WLAN) system. To do so, a WLAN system to which the presentinvention is applied will first be described in detail.

FIG. 1 is a diagram illustrating an exemplary configuration of a WLANsystem.

As illustrated in FIG. 1, the WLAN system includes at least one BasicService Set (BSS). The BSS is a set of STAs that are able to communicatewith each other by successfully performing synchronization.

An STA is a logical entity including a physical layer interface betweena Media Access Control (MAC) layer and a wireless medium. The STA mayinclude an AP and a non-AP STA. Among STAs, a portable terminalmanipulated by a user is the non-AP STA. If a terminal is simply calledan STA, the STA refers to the non-AP STA. The non-AP STA may also bereferred to as a terminal, a Wireless Transmit/Receive Unit (WTRU), aUser Equipment (UE), a Mobile Station (MS), a mobile terminal, or amobile subscriber unit.

The AP is an entity that provides access to a Distribution System (DS)to an associated STA through a wireless medium. The AP may also bereferred to as a centralized controller, a Base Station (BS), a Node-B,a Base Transceiver System (BTS), or a site controller.

The BSS may be divided into an infrastructure BSS and an Independent BSS(IBS S).

The BSS illustrated in FIG. 1 is the IBSS. The IBSS refers to a BSS thatdoes not include an AP. Since the IBSS does not include the AP, the IBSSis not allowed to access to the DS and thus forms a self-containednetwork.

FIG. 2 is a diagram illustrating another exemplary configuration of aWLAN system.

BSSs illustrated in FIG. 2 are infrastructure BSSs. Each infrastructureBSS includes one or more STAs and one or more APs. In the infrastructureBSS, communication between non-AP STAs is basically conducted via an AP.However, if a direct link is established between the non-AP STAs, directcommunication between the non-AP STAs may be performed.

As illustrated in FIG. 2, the multiple infrastructure BSSs may beinterconnected via a DS. The BSSs interconnected via the DS are calledan Extended Service Set (ESS). STAs included in the ESS may communicatewith each other and a non-AP STA within the same ESS may move from oneBSS to another BSS while seamlessly performing communication.

The DS is a mechanism that connects a plurality of APs to one another.The DS is not necessarily a network. As long as it provides adistribution service, the DS is not limited to any specific form. Forexample, the DS may be a wireless network such as a mesh network or maybe a physical structure that connects APs to one another.

Layer Architecture

An operation of an STA in a WLAN system may be described from theperspective of a layer architecture. A processor may implement the layerarchitecture in terms of device configuration. The STA may have aplurality of layers. For example, the 802.11 standards mainly deal witha MAC sublayer and a PHY layer on a Data Link Layer (DLL). The PHY layermay include a Physical Layer Convergence Protocol (PLCP) entity, aPhysical Medium Dependent (PMD) entity, and the like. Each of the MACsublayer and the PHY layer conceptually includes management entitiescalled MAC sublayer Management Entity (MLME) and Physical LayerManagement Entity (PLME). These entities provide layer managementservice interfaces through which a layer management function isexecuted.

To provide a correct MAC operation, a Station Management Entity residesin each STA. The SME is a layer independent entity which may beperceived as being present in a separate management plane or as beingoff to the side. While specific functions of the SME are not describedin detail herein, the SME may be responsible for collectinglayer-dependent states from various Layer Management Entities (LMEs) andsetting layer-specific parameters to similar values. The SME may executethese functions and implement a standard management protocol on behalfof general system management entities.

The above-described entities interact with one another in variousmanners. For example, the entities may interact with one another byexchanging GET/SET primitives between them. A primitive refers to a setof elements or parameters related to a specific purpose. AnXX-GET.request primitive is used to request a predetermined MIBattribute value (management information-based attribute information). AnX-GET.confirm primitive is used to return an appropriate MIB attributeinformation value when the Status field indicates “Success” and toreturn an error indication in the Status field when the Status fielddoes not indicate “Success”. An XX-SET.request primitive is used torequest setting of an indicated MIB attribute to a predetermined value.When the MIB attribute indicates a specific operation, the MIB attributerequests the specific operation to be performed. An XX-SET.confirrnprimitive is used to confirm that the indicated MIB attribute has beenset to a requested value when the Status field indicates “Success” andto return an error condition in the Status field when the Status fielddoes not indicate “Success”. When the MIB attribute indicates a specificoperation, it confirms that the operation has been performed.

Also, the MLME and the SME may exchange various MLME_GET/SET primitivesthrough an MLME Service Access Point (MLME_SAP). In addition, variousPLME_GET/SET primitives may be exchanged between the PLME and the SMEthrough a PLME_SAP, and exchanged between the MLME and the PLME throughan MLME-PLME_SAP.

Link Setup Process

FIG. 3 is a flowchart explaining a general link setup process accordingto an exemplary embodiment of the present invention.

In order to allow an STA to establish link setup on the network as wellas to transmit/receive data over the network, the STA must perform suchlink setup through processes of network discovery, authentication, andassociation, and must establish association and perform securityauthentication. The link setup process may also be referred to as asession initiation process or a session setup process. In addition, anassociation step is a generic term for discovery, authentication,association, and security setup steps of the link setup process.

Link setup process is described referring to FIG. 3.

In step S510, STA may perform the network discovery action. The networkdiscovery action may include the STA scanning action. That is, STA mustsearch for an available network so as to access the network. The STAmust identify a compatible network before participating in a wirelessnetwork. Here, the process for identifying the network contained in aspecific region is referred to as a scanning process.

The scanning scheme is classified into active scanning and passivescanning.

FIG. 3 is a flowchart illustrating a network discovery action includingan active scanning process. In the case of the active scanning, an STAconfigured to perform scanning transmits a probe request frame and waitsfor a response to the probe request frame, such that the STA can movebetween channels and at the same time can determine which Access Point(AP) is present in a peripheral region. A responder transmits a proberesponse frame, acting as a response to the probe request frame, to theSTA having transmitted the probe request frame. In this case, theresponder may be an STA that has finally transmitted a beacon frame in aBSS of the scanned channel. In BSS, since the AP transmits the beaconframe, the AP operates as a responder. In IBSS, since STAs of the IBSSsequentially transmit the beacon frame, the responder is not constant.For example, the STA, that has transmitted the probe request frame atChannel #1 and has received the probe response frame at Channel #1,stores BSS-associated information contained in the received proberesponse frame, and moves to the next channel (for example, Channel #2),such that the STA may perform scanning using the same method (i.e.,probe request/response transmission/reception at Channel #2).

Although not shown in FIG. 3, the scanning action may also be carriedout using passive scanning AN STA configured to perform scanning in thepassive scanning mode waits for a beacon frame while simultaneouslymoving from one channel to another channel. The beacon frame is one ofmanagement frames in IEEE 802.11, indicates the presence of a wirelessnetwork, enables the STA performing scanning to search for the wirelessnetwork, and is periodically transmitted in a manner that the STA canparticipate in the wireless network. In BSS, the AP is configured toperiodically transmit the beacon frame. In IBSS, STAs of the IBSS areconfigured to sequentially transmit the beacon frame. If each STA forscanning receives the beacon frame, the STA stores BSS informationcontained in the beacon frame, and moves to another channel and recordsbeacon frame information at each channel. The STA having received thebeacon frame stores BSS-associated information contained in the receivedbeacon frame, moves to the next channel, and thus performs scanningusing the same method.

In comparison between the active scanning and the passive scanning, theactive scanning is more advantageous than the passive scanning in termsof delay and power consumption.

After the STA discovers the network, the STA may perform theauthentication process in step S520. The authentication process may bereferred to as a first authentication process in such a manner that theauthentication process can be clearly distinguished from the securitysetup process of step S540.

The authentication process may include transmitting an authenticationrequest frame to an AP by the STA, and transmitting an authenticationresponse frame to the STA by the AP in response to the authenticationrequest frame. The authentication frame used for authenticationrequest/response may correspond to a management frame.

The authentication frame may include an authentication algorithm number,an authentication transaction sequence number, a state code, a challengetext, a Robust Security Network (RSN), a Finite Cyclic Group (FCG), etc.The above-mentioned information contained in the authentication framemay correspond to some parts of information capable of being containedin the authentication request/response frame, may be replaced with otherinformation, or may include additional information.

The STA may transmit the authentication request frame to the AP. The APmay decide whether to authenticate the corresponding STA on the basis ofinformation contained in the received authentication request frame. TheAP may provide the authentication result to the STA through theauthentication response frame.

After the STA has been successfully authenticated, the associationprocess may be carried out in step S530. The association process mayinvolve transmitting an association request frame to the AP by the STA,and transmitting an association response frame to the STA by the AP inresponse to the association request frame.

For example, the association request frame may include informationassociated with various capabilities, a beacon listen interval, aService Set Identifier (SSID), supported rates, supported channels, RSN,mobility domain, supported operating classes, a TIM (Traffic IndicationMap) broadcast request, interworking service capability, etc.

For example, the association response frame may include informationassociated with various capabilities, a state code, an Association ID(AID), supported rates, an Enhanced Distributed Channel Access (EDCA)parameter set, a Received Channel Power Indicator (RCPI), a ReceivedSignal to Noise Indicator (RSNI), mobility domain, a timeout interval(association comeback time), an overlapping BSS scan parameter, a TIMbroadcast response, a Quality of Service (QoS) map, etc.

The above-mentioned information may correspond to some parts ofinformation capable of being contained in the associationrequest/response frame, may be replaced with other information, or mayinclude additional information.

After the STA has been successfully associated with the network, asecurity setup process may be carried out in step S540. The securitysetup process of Step S540 may be referred to as an authenticationprocess based on Robust Security Network Association (RSNA)request/response. The authentication process of step S520 may bereferred to as a first authentication process, and the security setupprocess of Step S540 may also be simply referred to as an authenticationprocess.

For example, the security setup process of Step S540 may include aprivate key setup process through 4-way handshaking based on anExtensible Authentication Protocol over LAN (EAPOL) frame. In addition,the security setup process may also be carried out according to othersecurity schemes not defined in IEEE 802.11 standards.

Medium Access Mechanism

In the IEEE 802.11-based WLAN system, a basic access mechanism of MediumAccess Control (MAC) is a Carrier Sense Multiple Access with CollisionAvoidance (CSMA/CA) mechanism. The CSMA/CA mechanism is referred to as aDistributed Coordination Function (DCF) of IEEE 802.11 MAC, andbasically includes a “Listen Before Talk” access mechanism. Inaccordance with the above-mentioned access mechanism, the AP and/or STAmay perform Clear Channel Assessment (CCA) for sensing an RF channel ormedium during a predetermined time interval [for example, DCFInter-Frame Space (DIFS)], prior to data transmission. If it isdetermined that the medium is in the idle state, frame transmissionthrough the corresponding medium begins. On the other hand, if it isdetermined that the medium is in the occupied state, the correspondingAP and/or STA does not start its own transmission, establishes a delaytime (for example, a random backoff period) for medium access, andattempts to start frame transmission after waiting for a predeterminedtime. Through application of a random backoff period, it is expectedthat multiple STAs will attempt to start frame transmission afterwaiting for different times, resulting in minimum collision.

In addition, IEEE 802.11 MAC protocol provides a Hybrid CoordinationFunction (HCF). HCF is based on DCF and Point Coordination Function(PCF). PCF refers to the polling-based synchronous access scheme inwhich periodic polling is executed in a manner that all reception (Rx)APs and/or STAs can receive the data frame. In addition, HCF includesEnhanced Distributed Channel Access (EDCA) and HCF Controlled ChannelAccess (HCCA). EDCA is achieved when the access scheme provided from aprovider to a plurality of users is contention-based. HCCA is achievedby the contention-free-based channel access scheme based on the pollingmechanism. In addition, HCF includes a medium access mechanism forimproving Quality of Service (QoS) of WLAN, and may transmit QoS data inboth a Contention Period (CP) and a Contention Free Period (CFP).

FIG. 4 is a conceptual diagram illustrating a backoff process.

Operations based on a random backoff period will hereinafter bedescribed with reference to FIG. 4. If the occupy- or busy-state mediumis shifted to an idle state, several STAs may attempt to transmit data(or frame). As a method for implementing a minimum number of collisions,each STA selects a random backoff count, waits for a slot timecorresponding to the selected backoff count, and then attempts to startdata transmission. The random backoff count has a value of a PacketNumber (PN), and may be set to one of 0 to CW values. In this case, CWrefers to a Contention Window parameter value. Although an initial valueof the CW parameter is denoted by CWmin, the initial value may bedoubled in case of a transmission failure (for example, in the case inwhich ACK of the transmission frame is not received). If the CWparameter value is denoted by CWmax, CWmax is maintained until datatransmission is successful, and at the same time it is possible toattempt to start data transmission. If data transmission was successful,the CW parameter value is reset to CWmin Preferably, CW, CWmin, andCWmax are set to 2^(n)−1 (where n=0, 1, 2, . . . ).

If the random backoff process starts operation, the STA continuouslymonitors the medium while counting down the backoff slot in response tothe decided backoff count value. If the medium is monitored as theoccupied state, the countdown stops and waits for a predetermined time.If the medium is in the idle state, the remaining countdown restarts.

As shown in the example of FIG. 4, if a packet to be transmitted to MACof STA3 arrives at the STA3, the STA3 determines whether the medium isin the idle state during the DIFS, and may directly start frametransmission. In the meantime, the remaining STAs monitor whether themedium is in the busy state, and wait for a predetermined time. Duringthe predetermined time, data to be transmitted may occur in each ofSTA1, STA2, and STA5. If the medium is in the idle state, each STA waitsfor the DIFS time and then performs countdown of the backoff slot inresponse to a random backoff count value selected by each STA. Theexample of FIG. 4 shows that STA2 selects the lowest backoff count valueand STA1 selects the highest backoff count value. That is, after STA2finishes backoff counting, the residual backoff time of STA5 at a frametransmission start time is shorter than the residual backoff time ofSTA1. Each of STA1 and STA5 temporarily stops countdown while STA2occupies the medium, and waits for a predetermined time. If occupying ofthe STA2 is finished and the medium re-enters the idle state, each ofSTA1 and STA5 waits for a predetermined time DIFS, and restarts backoffcounting. That is, after the remaining backoff slot as long as theresidual backoff time is counted down, frame transmission may startoperation. Since the residual backoff time of STA5 is shorter than thatof STA1, STA5 starts frame transmission. Meanwhile, data to betransmitted may occur in STA4 while STA2 occupies the medium. In thiscase, if the medium is in the idle state, STA4 waits for the DIFS time,performs countdown in response to the random backoff count valueselected by the STA4, and then starts frame transmission. FIG. 4exemplarily shows the case in which the residual backoff time of STA5 isidentical to the random backoff count value of STA4 by chance. In thiscase, an unexpected collision may occur between STA4 and STA5. If thecollision occurs between STA4 and STA5, each of STA4 and STA5 does notreceive ACK, resulting in the occurrence of a failure in datatransmission. In this case, each of STA4 and STA5 increases the CW valuetwo times, and STA4 or STA5 may select a random backoff count value andthen perform countdown. Meanwhile, STA1 waits for a predetermined timewhile the medium is in the occupied state due to transmission of STA4and STA5. In this case, if the medium is in the idle state, STA1 waitsfor the DIFS time, and then starts frame transmission after lapse of theresidual backoff time.

STA Sensing Operation

As described above, the CSMA/CA mechanism includes not only a physicalcarrier sensing mechanism in which the AP and/or STA can directly sensethe medium, but also a virtual carrier sensing mechanism. The virtualcarrier sensing mechanism can solve some problems (such as a hidden nodeproblem) encountered in the medium access. For the virtual carriersensing, MAC of the WLAN system can utilize a Network Allocation Vector(NAV). In more detail, by means of the NAV value, the AP and/or STA,each of which currently uses the medium or has authority to use themedium, may inform another AP and/or another STA for the remaining timein which the medium is available. Accordingly, the NAV value maycorrespond to a reserved time in which the medium will be used by the APand/or STA configured to transmit the corresponding frame. AN STA havingreceived the NAV value may prohibit medium access (or channel access)during the corresponding reserved time. For example, NAV may be setaccording to the value of a ‘duration’ field of the MAC header of theframe.

The robust collision detect mechanism has been proposed to reduce theprobability of such collision, and as such a detailed descriptionthereof will hereinafter be described with reference to FIGS. 7 and 8.Although an actual carrier sensing range is different from atransmission range, it is assumed that the actual carrier sensing rangeis identical to the transmission range for convenience of descriptionand better understanding of the present invention.

FIG. 5 is a conceptual diagram illustrating a hidden node and an exposednode.

FIG. 5(a) exemplarily shows the hidden node. In FIG. 5(a), STA Acommunicates with STA B, and STA C has information to be transmitted. InFIG. 5(a), STA C may determine that the medium is in the idle state whenperforming carrier sensing before transmitting data to STA B, under thecondition that STA A transmits information to STA B. Since transmissionof STA A (i.e., occupied medium) may not be detected at the location ofSTA C, it is determined that the medium is in the idle state. In thiscase, STA B simultaneously receives information of STA A and informationof STA C, resulting in the occurrence of collision. Here, STA A may beconsidered as a hidden node of STA C.

FIG. 5(b) exemplarily shows an exposed node. In FIG. 5(b), under thecondition that STA B transmits data to STA A, STA C has information tobe transmitted to STA D. If STA C performs carrier sensing, it isdetermined that the medium is occupied due to transmission of STA B.Therefore, although STA C has information to be transmitted to STA D,the medium-occupied state is sensed, such that the STA C must wait for apredetermined time (i.e., standby mode) until the medium is in the idlestate. However, since STA A is actually located out of the transmissionrange of STA C, transmission from STA C may not collide withtransmission from STA B from the viewpoint of STA A, such that STA Cunnecessarily enters the standby mode until STA B stops transmission.Here, STA C is referred to as an exposed node of STA B.

FIG. 6 is a conceptual diagram illustrating Request To Send (RTS) andClear To Send (CTS).

In order to efficiently utilize the collision avoidance mechanism underthe above-mentioned situation of FIG. 5, it is possible to use a shortsignaling packet such as RTS and CTS. RTS/CTS between two STAs may beoverheard by peripheral STA(s), such that the peripheral STA(s) mayconsider whether information is communicated between the two STAs. Forexample, if STA to be used for data transmission transmits the RTS frameto the STA having received data, the STA having received data transmitsthe CTS frame to peripheral STAs, and may inform the peripheral STAsthat the STA is going to receive data.

FIG. 6(a) exemplarily shows the method for solving problems of thehidden node. In FIG. 6(a), it is assumed that each of STA A and STA C isready to transmit data to STA B. If STA A transmits RTS to STA B, STA Btransmits CTS to each of STA A and STA C located in the vicinity of theSTA B. As a result, STA C must wait for a predetermined time until STA Aand STA B stop data transmission, such that collision is prevented fromoccurring.

FIG. 6(b) exemplarily shows the method for solving problems of theexposed node. STA C performs overhearing of RTS/CTS transmission betweenSTA A and STA B, such that STA C may determine no collision although ittransmits data to another STA (for example, STA D). That is, STA Btransmits an RTS to all peripheral STAs, and only STA A having data tobe actually transmitted can transmit a CTS. STA C receives only the RTSand does not receive the CTS of STA A, such that it can be recognizedthat STA A is located outside of the carrier sensing range of STA C.

Power Management

As described above, the WLAN system has to perform channel sensingbefore STA performs data transmission/reception. The operation of alwayssensing the channel causes persistent power consumption of the STA.There is not much difference in power consumption between the Reception(Rx) state and the Transmission (Tx) state. Continuous maintenance ofthe Rx state may cause large load to a power-limited STA (i.e., STAoperated by a battery). Therefore, if STA maintains the Rx standby modeso as to persistently sense the channel, power is inefficiently consumedwithout special advantages in terms of WLAN throughput. In order tosolve the above-mentioned problem, the WLAN system supports a PowerManagement (PM) mode of the STA.

The PM mode of the STA is classified into an active mode and a PowerSave (PS) mode. The STA is basically operated in the active mode. TheSTA operating in the active mode maintains an awake state. If the STA isin the awake state, the STA may normally operate such that it canperform frame transmission/reception, channel scanning, or the like. Onthe other hand, STA operating in the PS mode is configured to switchfrom the doze state to the awake state or vice versa. STA operating inthe sleep state is operated with minimum power, and the STA does notperform frame transmission/reception and channel scanning.

The amount of power consumption is reduced in proportion to a specifictime in which the STA stays in the sleep state, such that the STAoperation time is increased in response to the reduced powerconsumption. However, it is impossible to transmit or receive the framein the sleep state, such that the STA cannot mandatorily operate for along period of time. If there is a frame to be transmitted to the AP,the STA operating in the sleep state is switched to the awake state,such that it can transmit/receive the frame in the awake state. On theother hand, if the AP has a frame to be transmitted to the STA, thesleep-state STA is unable to receive the frame and cannot recognize thepresence of a frame to be received. Accordingly, STA may need to switchto the awake state according to a specific period in order to recognizethe presence or absence of a frame to be transmitted to the STA (or inorder to receive a signal indicating the presence of the frame on theassumption that the presence of the frame to be transmitted to the STAis decided).

The AP may transmit a beacon frame to STAs in a BSS at predeterminedintervals. The beacon frame may include a traffic indication map (TIM)information element. The TIM information element may include informationindicating that the AP has buffered traffic for STAs associatedtherewith and will transmit frames. TIM elements include a TIM used toindicate a unitcast frame and a delivery traffic indication map (DTIM)used to indicate a multicast or broadcast frame.

FIGS. 7 to 9 are conceptual diagrams illustrating detailed operations ofthe STA having received a Traffic Indication Map (TIM).

Referring to FIG. 7, STA is switched from the sleep state to the awakestate so as to receive the beacon frame including a TIM from the AP. STAinterprets the received TIM element such that it can recognize thepresence or absence of buffered traffic to be transmitted to the STA.After STA contends with other STAs to access the medium for PS-Pollframe transmission, the STA may transmit the PS-Poll frame forrequesting data frame transmission to the AP. The AP having received thePS-Poll frame transmitted by the STA may transmit the frame to the STA.STA may receive a data frame and then transmit an ACK frame to the AP inresponse to the received data frame. Thereafter, the STA may re-enterthe sleep state.

As can be seen from FIG. 7, the AP may operate according to theimmediate response scheme, such that the AP receives the PS-Poll framefrom the STA and transmits the data frame after lapse of a predeterminedtime [for example, Short Inter-Frame Space (SIFS)]. In contrast, the APhaving received the PS-Poll frame does not prepare a data frame to betransmitted to the STA during the SIFS time, such that the AP mayoperate according to the deferred response scheme, and as such adetailed description thereof will hereinafter be described withreference to FIG. 8.

The STA operations of FIG. 8 in which the STA is switched from the sleepstate to the awake state, receives a TIM from the AP, and transmits thePS-Poll frame to the AP through contention are identical to those ofFIG. 7. If the AP having received the PS-Poll frame does not prepare adata frame during the SIFS time, the AP may transmit the ACK frame tothe STA instead of transmitting the data frame. If the data frame isprepared after transmission of the ACK frame, the AP may transmit thedata frame to the STA after completion of such contending. STA maytransmit the ACK frame indicating successful reception of a data frameto the AP, and may be shifted to the sleep state.

FIG. 9 shows the exemplary case in which AP transmits DTIM. STAs may beswitched from the sleep state to the awake state so as to receive thebeacon frame including a DTIM element from the AP. STAs may recognizethat multicast/broadcast frame(s) will be transmitted through thereceived DTIM. After transmission of the beacon frame including theDTIM, AP may directly transmit data (i.e., multicast/broadcast frame)without transmitting/receiving the PS-Poll frame. While STAscontinuously maintains the awake state after reception of the beaconframe including the DTIM, the STAs may receive data, and then switch tothe sleep state after completion of data reception.

Frame Structure

FIG. 10 is an explanatory diagram of an exemplary frame structure usedin an IEEE 802.11 system.

A PPDU (Physical Layer Protocol Data Unit) frame format may include anSTF (Short Training Field), an LTF (Long Training Field), a SIG (SIGNAL)field and a data field. The most basic (e.g., non-HT (High Throughput))PPDU frame format may include only an L-STF (Legacy-STF), an L-LTF(Legacy-LTF), a SIG field and a data field.

The STF is a signal for signal detection, AGC (Automatic Gain Control),diversity selection, accurate time synchronization, etc., and the LTF isa signal for channel estimation, frequency error estimation, etc. TheSTF and LTF may be collectively called a PLCP preamble. The PLCPpreamble may be regarded as a signal for OFDM physical layersynchronization and channel estimation.

The SIG field may include a RATE field and a LENGTH field. The RATEfield may include information about modulation and coding rates of data.The LENGTH field may include information about the length of data. Inaddition, the SIG field may include a parity bit, a SIG TAIL bit, etc.

The data field may include a SERVICE field, a PSDU (Physical layerService Data Unit) and a PPDU TAIL bit. The data field may also includepadding bits as necessary. Some bits of the SERVICE field may be usedfor synchronization of a descrambler at a receiving end. The PSDUcorresponds to an MPDU (MAC Protocol Data Unit) defined in the MAC layerand may include data generated/used in a higher layer. The PPDU TAIL bitmay be used to return an encoder to state 0. The padding bits may beused to adjust the length of the data field to a predetermined unit.

The MPDU is defined depending on various MAC frame formats, and a basicMAC frame includes a MAC header, a frame body and an FCS (Frame CheckSequence). The MAC frame may be composed of the MPDU andtransmitted/received through PSDU of a data part of the PPDU frameformat.

The MAC header includes a frame control field, a duration/ID field, anaddress field, etc. The frame control field may include controlinformation necessary for frame transmission/reception. The duration/IDfield may be set to a time to transmit a relevant a relevant frame.

The duration/ID field included in the MAC header may be set to a 16-bitlength (e.g., B0 to B15). Content included in the duration/ID field maydepend on frame type and sub-type, whether transmission is performed fora CFP (contention free period), QoS capability of a transmission STA andthe like. (i) In a control frame corresponding to a sub-type of PS-Poll,the duration/ID field may include the AID of the transmission STA (e.g.,through 14 LSBs) and 2 MSBs may be set to 1. (ii) In frames transmittedby a PC (point coordinator) or a non-QoS STA for a CFP, the duration/IDfield may be set to a fixed value (e.g., 32768). (iii) In other framestransmitted by a non-QoS STA or control frames transmitted by a QoS STA,the duration/ID field may include a duration value defined per frametype. In a data frame or a management frame transmitted by a QoS STA,the duration/ID field may include a duration value defined per frametype. For example, B15=0 of the duration/ID field indicates that theduration/ID field is used to indicate a TXOP duration, and B0 to B14 maybe used to indicate an actual TXOP duration. The actual TXOP durationindicated by B0 to B14 may be one of 0 to 32767 and the unit thereof maybe microseconds (μs). However, when the duration/ID field indicates afixed TXOP duration value (e.g., 32768), B15 can be set to 1 and B0 toB14 can be set to 0. When B14=1 and B15=1, the duration/ID field is usedto indicate an AID, and B0 to B13 indicate one AID of 1 to 2007. Referto the IEEE 802.11 standard document for details of Sequence Control,QoS Control, and HT Control subfields of the MAC header.

The frame control field of the MAC header may include Protocol Version,Type, Subtype, To DS, From DS, More Fragment, Retry, Power Management,More Data, Protected Frame and Order subfields. Refer to the IEEE 802.11standard document for contents of the subfields of the frame controlfield.

FIG. 11 illustrates a CF (contention free)-END frame.

It is assumed that the CF-END frame is transmitted by a non-DMG(directional multi-gigabit, 11ad) STA for convenience of description.The CF-END frame may be transmitted to truncate a TXOP duration.Accordingly, a duration field is set to 0 in the CF-END frame. An RA(Receiver Address) field may be set to a broadcast group address. ABSSID field may be set to an STA address included in a relevant AP.However, in the case of a CF-END frame in a non-HT or non-HT duplicateformat, which is transmitted from a VHT STA to a VHT AP, anIndividual/Group bit of the BSSID field may be set to 1.

Example of HE PPDU Structure

A description will be given of examples of an HE PPDU (High EfficiencyPhysical layer Protocol Data Unit) format in a wireless LAN systemsupporting 11ax.

FIG. 12 illustrates an example of the HE PPDU. Referring to FIG. 12, anHE-SIG A (or HE-SIG1) field follows an L-Part (e.g., L-STF, L-LTF,L-SIG) and is duplicated every 20 MHz like the L-Part. The HE-SIG Afield includes common control information (e.g., BW, GI length, BSSindex, CRC, Tail, etc.) for STAs. The HE-SIG A field includesinformation for decoding the HE PPDU and thus information included inthe HE-SIG A field may depend on the format of the HE PPDU (e.g., SUPPDU, MU PPDU, trigger-based PPDU or the like). For example, in the HESU PPDU format, the HE-SIG A field may include at least one of a DL/ULindicator, HE PPDU format indicator, BSS color, TXOP duration, BW(bandwidth), MCS, CP+LTF length, coding information, the number ofstreams, STBC (e.g., whether STBC is used), transmission beamforming(TxBF) information, CRC and Tail. In the case of the HE SU PPDU format,the HE-SIG B field may be omitted. In the HE MU PPDU format, the HE-SIGA field may include at least one of a DL/UL indicator, BSS color, TXOPduration, BW, MCS information of a SIG B field, the number of symbols ofthe SIG B field, the number of HE LTF symbols, indicator indicatingwhether full band MU-MIMO is used, CP+LTF length, transmissionbeamforming (TxBF) information, CRC and Tail. In the HE trigger-basedPPDU format, an HE-SIG A field may include at least one of a formatindicator (e.g., indicating the SU PPDU or trigger-based PPDU), BSScolor, TXOP duration, BW, CRC and Tail.

FIG. 13 illustrates another example of the HE PPDU. Referring to FIG.13, the HE-SIG A may include user allocation information, for example,at least one of an STA ID such as a PAID or a GID, allocated resourceinformation and the number of streams (Nsts), in addition to the commoncontrol information. Referring to FIG. 13, the HE-SIG B (or HE-SIG2) maybe transmitted for each OFDMA allocation. In the case of MU-MIMO, theHE-SIG B is identified by an STA through SDM. The HE-SIG B may includeadditional user allocation information, for example, an MCS, codinginformation, STBC (Space Time Block Code) information and transmissionbeamforming (TXBF) information.

FIG. 14 illustrates another example of the HE PPDU. The HE-SIG B istransmitted following the HE-SIG A. The HE-SIG B may be transmittedthrough the full band on the basis of numerology of the HE-SIG A. TheHE-SIG B may include user allocation information, for example, STA AID,resource allocation information (e.g., allocation size), MCS, the numberof streams (Nsts), coding, STBC and transmission beamforming (TXBF)information.

FIG. 15 illustrates another example of the HE PPDU. The HE-SIG B may beduplicated per predetermined unit channel Referring to FIG. 15, theHE-SIG B may be duplicated per 20 MHz. For example, the HE-SIG B can betransmitted in such a manner that the same information is duplicated per20 MHz in 80 MHz bandwidth.

An STA/AP which has received the HE-SIG B duplicated every 20 MHz mayaccumulate the received HE-SIG B per 20 MHz channel to improvereliability of HE-SIG B reception.

Since the same signal (e.g., HE-SIG B) is duplicated and transmitted perchannel, the gain of accumulated signals is proportional to the numberof channels over which the signal is duplicated and transmitted toimprove reception performance. In theory, a duplicated and transmittedsignal can have a gain corresponding to 3 dB×(the number of channels)compared to the signal before duplication. Accordingly, the duplicatedand transmitted HE-SIG B may be transmitted with an increased MCS leveldepending on the number of channels through which the HE-SIG B isduplicated and transmitted. For example, if MCS0 is used for the HE-SIGB transmitted without being duplicated, MCS1 can be used for the HE-SIGB duplicated and transmitted. Since the HE-SIG B can be transmitted witha higher MCS level as the number of channels for duplication increases,HE-SIG B overhead per unit channel can be reduced.

FIG. 16 illustrates another example of the HE PPDU. Referring to FIG.16, the HE-SIG B may include independent information per 20 MHz channel.The HE-SIG B may be transmitted in a 1× symbol structure like the Legacypart (e.g., L-STF, L-LTF, L-SIG) and HE-SIG A. Meanwhile, a length of“L-STF+L-LTF+L-SIG+HE-SIGA+HE-SIGB” needs to be identical in allchannels in a wide bandwidth. The HE-SIG B transmitted per 20 MHzchannel may include allocation information about the corresponding band,for example, allocation information per user using the correspondingband, user ID, etc. However, the information of the HE-SIG B may varybetween bands because the respective bands support different numbers ofusers and use different resource block configurations. Accordingly, thelength of the HE-SIG B may be different for respective channels.

FIG. 17 illustrates an HE-SIG B padding method by which lengths beforeHE-STF (e.g., lengths to the HE-SIG B) become identical for respectivechannels. For example, the HE-SIG B may be duplicated by a paddinglength to align HE-SIG B lengths. As illustrated in FIG. 18, the HE-SIGB corresponding to a necessary padding length may be padded to theHE-SIG B from the start (or end) of the HE-SIG B.

According to an example, one HE-SIG B field can be transmitted when thebandwidth does not exceed 20 MHz. When the bandwidth exceeds 20 MHz, 20MHz channels may respectively transmit one of a first type HE-SIG B(referred to hereinafter as HE-SIG B [1]) and a second type HE-SIG B(referred to hereinafter as HE-SIG B [2]). For example, HE-SIG B [1] andHE-SIG B [2] may be alternately transmitted. An odd-numbered 20 MHzchannel may deliver HE-SIG B [1] and an even-numbered 20 MHz channel maydeliver HE-SIG B [2]. More specifically, in the case of a 40 MHzbandwidth, HE-SIG B [1] is transmitted over the first 20 MHz channel andHE-SIG B [2] is transmitted over the second 20 MHz channel. In the caseof an 80 MHz bandwidth, HE-SIG B [1] is transmitted over the first 20MHz channel, HE-SIG B [2] is transmitted over the second 20 MHz channel,the same HE-SIG B [1] is duplicated and transmitted over the third 20MHz channel and the same HE-SIG B [2] is duplicated and transmitted overthe fourth 20 MHz channel. The HE-SIG B is transmitted in a similarmanner in the case of a 160 MHz bandwidth.

As described above, the HE-SIG B can be duplicated and transmitted asthe bandwidth increases. Here, a duplicated HE-SIG B may befrequency-hopped by 20 MHz from a 20 MHz channel over which an HE-SIG Bof the same type is transmitted and transmitted.

HE-SIG B [1] and HE-SIG B [2] may have different content. However,HE-SIG-Bs [1] have the same content. Similarly, HE-SIG Bs [2] have thesame content.

According to an embodiment, HE-SIG B [1] may be configured to includeresource allocation information about only odd-numbered 20 MHz channelsand HE-SIG B [2] may be configured to include resource allocationinformation about only even-numbered 20 MHz channels. According toanother embodiment of the present invention, HE-SIG B [1] may includeresource allocation information about at least part of even-numbered 20MHz channels or HE-SIG B [2] may include resource allocation informationabout at least part of odd-numbered 20 MHz channels.

The HE-SIG B may include a common field and a user-specific field. Thecommon field may precede the user-specific field. The common field andthe user-specific field may be distinguished in a unit of bit(s) insteadof a unit of OFDM symbol(s).

The common field of the HE-SIG B includes information for all STAsdesignated to receive PPDUs in a corresponding bandwidth. The commonfield may include resource unit (RU) allocation information. All theHE-SIG Bs [1] may have the same content and All the HE-SIG Bs [2] mayhave the same content. For example, when four 20 MHz channelsconstituting 80 MHz are classified as [LL, LR, RL, RR], the common fieldof HE-SIG B [1] may include a common block for LL and RL and the commonfield of HE-SIG B [2] may include a common block for LR and RR.

The user-specific field of the HE-SIG B may include a plurality of userfields. Each user field may include information specific to anindividual STA designated to receive PPDUs. For example, the user fieldmay include at least one of an STA ID, MCS per STA, the number ofstreams (Nsts), coding (e.g., indication of use of LDPC), DCM indicatorand transmission beamforming information. However, the information ofthe user field is not limited thereto.

UL MU Transmission

FIG. 19 is an explanatory diagram of an uplink multi-user transmissionsituation according to an embodiment of the present invention.

As described above, an 802.11ax system may employ UL MU transmission. ULMU transmission may be started when an AP transmits a trigger frame to aplurality of STAs (e.g., STA1 to STA4), as illustrated in FIG. 19. Thetrigger frame may include UL MU allocation information. The UL MUallocation information may include at least one of resource position andsize, STA IDs or reception STA addresses, MCS and MU type (MIMO, OFDMA,etc.). Specifically, the trigger frame may include at least one of (i) aUL MU frame duration, (ii) the number of allocations (N) and (iii)information per allocation. The information per allocation may includeinformation per user (Per user Info). The information per allocation mayinclude at least one of an AID (AIDs corresponding to the number of STAsare added in the case of MU), power adjustment information, resource (ortone) allocation information (e.g., bitmap), MCS, the number of streams(Nsts), STBC, coding and transmission beamforming information.

As illustrated in FIG. 19, the AP may acquire TXOP to transmit thetrigger frame through a contention procedure to access media.Accordingly, the STAs may transmit UL data frames in a format indicatedby the AP after SIFS of the trigger frame. It is assumed that the APaccording to an embodiment of the present invention sends anacknowledgement response to the UL data frames through a block ACK (BA)frame.

FIG. 20 illustrates a trigger frame format according to an embodiment.

Referring to FIG. 20, the trigger frame may include at least one of aframe control field, a duration field, an RA (recipient STA address)field, a TA (transmitting STA address) field, a common informationfield, one or more Per User Info fields and FCS (Frame Check Sum). TheRA field indicates the address or ID of a recipient STA and may beomitted according to embodiments. The TA field indicates the address ofa transmitting STA.

The common information field may include at least one of a lengthsubfield, a cascade indication subfield, an HE-SIG A informationsubfield, a CP/LTF type subfield, a trigger type subfield and atrigger-dependent common information subfield. The length subfieldindicates the L-SIG length of a UL MU PPDU. The cascade indicationindicates whether there is transmission of a subsequent trigger framefollowing the current trigger frame. The HE-SIG A information subfieldindicates content to be included in the HE-SIG A of the UL MU PPDU. TheCP/LTF type subfield indicates a CP and HE LTF type included in the ULMU PPDU. The trigger type subfield indicates the type of the triggerframe. The trigger frame may include common information specific to thetype and information per user (Per User Info) specific to the type. Forexample, the trigger type may be set to one of a basic trigger type(e.g., type 0), beamforming report poll trigger type (e.g., type 1),MU-BAR (Multi-user Block Ack Request) type (e.g., type 2) and MU-RTS(multi-user ready to send) type (e.g., type 3). However the trigger typeis not limited thereto. When the trigger type is MU-BAR, thetrigger-dependent common information subfield may include a GCR(Groupcast with Retries) indicator and a GCR address.

The Per User Info field may include at least one of a user ID subfield,an RU allocation subfield, a coding type subfield, an MCS subfield, aDCM (dual sub-carrier modulation) subfield, an SS (spatial stream)allocation subfield and a trigger dependent Per User Info subfield. Theuser ID subfield indicates the AID of an STA which will use acorresponding resource unit to transmit MPDU of the UL MU PPDU. The RUallocation subfield indicates a resource unit used for the STA totransmit the UL MU PPDU. The coding type subfield indicates the codingtype of the UL MU PPDU transmitted by the STA. The MCS subfieldindicates the MCS of the UL MU PPDU transmitted by the STA. The DCMsubfield indicates information about double carrier modulation of the ULMU PPDU transmitted by the STA. The SS allocation subfield indicatesinformation about spatial streams of the UL MU PPDU transmitted by theSTA. In the case of MU-BAR trigger type, the trigger-dependent Per UserInfo subfield may include BAR control and BAR information.

NAV (Network Allocation Vector)

A NAV may be understood as a timer for protecting TXOP of a transmittingSTA (e.g., TXOP holder). An STA may not perform channel access during aperiod in which a NAV configured in the STA is valid so as to protectTXOP of other STAs.

A current non-DMG STA supports one NAV. An STA which has received avalid frame can update the NAV through the duration field of the PSDU(e.g., the duration field of the MAC header). When the RA field of thereceived frame corresponds to the MAC address of the STA, however, theSTA does not update the NAV. When a duration indicated by the durationfield of the received frame is greater than the current NAV value of theSTA, the STA updates the NAV through the duration of the received frame.

FIG. 21 illustrates an example of NAV setting.

Referring to FIG. 21, a source STA transmits an RTS frame and adestination STA transmits CTS frame. As described above, the destinationSTA designated as a recipient through the RTS frame does not set a NAV.Some of other STAs may receive the RTS frame and set NAVs and others mayreceive the CTS frame and set NAVs.

If the CTS frame (e.g., PHY-RXSTART.indication primitive) is notreceived within a predetermined period from a timing when the RTS frameis received (e.g., PHY-RXEND.indication primitive for which MACcorresponds to the RTS frame is received), STAs which have set orupdated NAVs through the RTS frame can reset the NAVs (e.g., 0). Thepredetermined period may be(2*aSIFSTime+CTS_Time+aRxPHYStartDelay+2*aSlotTime). The CTS_Time may becalculated on the basis of the CTS frame length indicated by the RTSframe and a data rate.

Although FIG. 21 illustrates setting or update of a NAV through the RTSframe or CTS frame for convenience, NAV setting/resetting/update may beperformed on the basis of duration fields of various frames, forexample, non-HT PPDU, HT PPDU, VHT PPDU and HE PPDU (e.g., the durationfield of the MAC header of the MAC frame). For example, if the RA fieldof the received MAC frame does not correspond to the address of an STA(e.g., MAC address), the STA may set/reset/update the NAV.

TXOP (Transmission Opportunity) Truncation

FIG. 22 illustrates an example of TXOP truncation.

A TXOP holder STA may indicate to truncate TXOP by transmitting a CF-ENDframe. AN STA can reset the NAV (e.g., set the NAV to 0) upon receptionof a CF-END frame or CF-END+CF-ACK frame.

When an STA that has acquired channel access through EDCA empties atransmission queue thereof, the STA can transmit a CF-END frame. The STAcan explicitly indicate completion of TXOP thereof through transmissionof the CF-END frame. The CF-END frame may be transmitted by a TXOPholder. A non-AP STA that is not a TXOP holder cannot transmit theCF-END frame. A STA which has received the CF-END frame resets the NAVat a time when a PPDU included in the CF-END frame is ended.

Referring to FIG. 22, an STA that has accessed a medium transmits asequence (e.g., RTS/CTS) for NAV setting.

After SIFS, a TXOP holder (or TXOP initiator) and a TXOP respondertransmit and receive PPDUs (e.g., initiator sequence). The TXOP holdertruncates a TXOP by transmitting a CF-END frame when there is no data tobe transmitted within the TXOP.

STAs which have received the CF-END frame reset NAYS thereof and canstart contending for medium access without delay.

As described above, a TXOP duration is set through the duration field ofthe MAC header in the current wireless LAN system. That is, a TXOPholder (e.g., Tx STA) and a TXOP responder (e.g., Rx STA) include wholeTXOP information necessary for transmission and reception of frames induration fields of frames transmitted and received therebetween andtransmit the frames. Third party STAs other than the TXOP holder and theTXOP responder check the duration fields of frames exchanged between theTXOP holder and the TXOP responder and sets/updates NAVs to defer use ofchannels until NAV periods.

In an 11ax system supporting the HE PPDU, the third party STAs cannotdecode an MPDU included in a UL MU PPDU even when they receive the UL MUPPDU if the UL MU PPDU does not include the HE-SIG B. If the third partySTAs cannot decode the MPDU, the third party STAs cannot acquire TXOPduration information (e.g., duration field) included in the MAC headerof the MPDU. Accordingly, it is difficult to correctly perform NAVsetting/update.

Even when an HE PPDU frame including the HE-SIG B is received, if theHE-SIG B structure is encoded per STA and is designed such that a STAcan read only HE-SIG B content allocated to that STA, the third partySTAs cannot decode a MAC frame (e.g., an MPDU in the HE PPDUcorresponding to other STAS) transmitted and received by other STAs.Accordingly, the third party STAs cannot acquire TXOP information inthis case.

TXOP Duration Indication through HE-SIG A

To solve the aforementioned problem, a method through which an STAincludes TXOP duration information in the HE-SIG A and transmits theHE-SIG A is proposed. As described above, 15 bits (e.g., B0 to B14) ofthe duration field of the MAC header may indicate duration informationof up to 32.7 ms (0 to 32767 us). When the 15-bit duration informationincluded in the duration field of the MAC header is included in theHE-SIG A and transmitted, an 11ax third party STA can correctlyset/update a NAV. However, HE-SIG A signaling overhead excessivelyincreases. While 15 bits in an MPDU for payload transmission can beregarded as a relatively small size in the MAC layer, the HE-SIG A forcommon control information transmission in the physical layer is acompactly designed field, and thus an increase of 15 bits in the HE-SIGA corresponds to relatively large signaling overhead.

Accordingly, an embodiment of the present invention proposes anefficient TXOP duration indication method for minimizing HE-SIG Aoverhead. In addition, an embodiment of the present invention proposesframe transmission and reception operations based on a TXOP durationnewly defined in the HE-SIG A. Hereinafter, the duration field includedin the MAC header may be referred to as a MAC duration for convenience.

While it is assumed that TXOP duration information is included in the HESIG A and transmitted in the following description, the scope of thepresent invention is not limited thereto and the TXOP durationinformation may be transmitted through other parts (e.g., L-SIG, HE-SIGB, HE-SIG C, . . . , and part of A-MPDU or MPDU). For example, when aTXOP duration is transmitted through the HE-SIG B, the TXOP duration canbe transmitted through common information (e.g., common part) of theHE-SIG B or a SIG B contents part (e.g., Per user Info) transmitted atthe first (or end) part of the HE-SIG B.

A description will be given of a TXOP duration structure in an HE SIGfield and examples indicating the TXOP duration. A value set to the NAVof a third party STA can be interpreted as a TXOP duration for a TXOPholder/responder. For example, a duration field value is a TXOP forframe transmission and reception in view of the TXOP holder/responder.However, the duration field value refers to a NAV value in view of thethird party STA. Accordingly, a NAV setting/update operation of thethird party STA may be referred to as a TXOP setting/update operationbecause the NAV setting/update operation sets a NAV corresponding to aTXOP for the TXOP holder/responder. Furthermore, the term “TXOPduration” may be simply referred to as “duration” or “TXOP”. The TXOPduration may be used to indicate a field (e.g., the TXOP duration fieldof the HE-SIG A) in a frame or to indicate an actual TXOP durationvalue.

Indices assigned to examples described below are for convenience ofdescription and thus examples having different indices may be combinedto embody one invention or respective examples may embody respectiveinventions.

Example 1

The TXOP duration may be set to 2^(N)−1 (or 2^(N)). It is assumed thatthe TXOP duration is set to 2^(N)−1 for convenience. The value N can betransmitted in the TXOP duration field of the HE-SIG A.

For example, when N is 4 bits, N has a value in the range of 0 to 15.Accordingly, the TXOP duration indicated through N having a size of 4bits may have a value in the range of 0 to 32,767 μs. When the TXOPduration is set to indicate a maximum of 5 ms, only N=0 to 13 may beused to indicate the TXOP duration and N=14 and N=15 may be used forother purposes.

This example is one of methods for indicating the TXOP duration throughX*2^(Y)−1 (e.g., X=1), X and/or Y may be changed in various manners. Inaddition, values X and Y may be transmitted through the HE-SIG A field.

Example 2

According to an embodiment of the present invention, the TXOP durationmay be set to X^(Y)−1 (or X^(Y)). It is assumed that the TXOP durationis set to X^(Y)−1. AN STA can transmit values X and Y through the TXOPduration field (e.g., in the HE-SIG A).

If the TXOP duration field transmitted in the HE-SIG A is K bits, n bits(first n bits) of the K bits may indicate the value X and m bits thereof(e.g., m bits at the end) may indicate the value Y. The n bits may be nMSBs or n LSBs and the m bits may be m LSBs or m MSBs. The values K, mand n can be set in various manners.

(i) For example, it is assumed that K=6, n=3 and m=3. When X␣{2˜9} andY□{0˜7}, the TXOP duration may have a value in the range of 0 to4,782,968 μs.

(ii) In another example, it is assumed that K=5, n=2 and m=3. WhenX□{2˜5} and Y□{0˜7}, the TXOP duration may have a value in the range of0 to 78,124 μs. If X□{2, 3, 5, 6} and Y□{0˜7}, the TXOP duration mayhave a value in the range of 0 to 78,124 μs.

(iii) In another example, it is assumed that K=4, n=1 and m=3. WhenX□{2, 3} (or X□{5, 6}) and Y□{0˜7}, the TXOP duration may have a valuein the range of 0 to 279,963 μs.

If a maximum of P ms (e.g., 5 ms) is indicated through the TXOP durationfield (e.g., in the HE-SIG A), an (X, Y) combination that minimizesX^(Y)−1, from among (X, Y) combinations satisfying X^(Y)−1≥P ms (e.g., 5ms), may be used to indicate a maximum TXOP duration value and other (X,Y) combinations may not be used.

This example is one of methods of indicating the TXOP duration throughZ*X^(Y)−1 and thus X, Y and/or Z may be changed in various manners.

Example 3

According to an embodiment of the present invention, the TXOP durationmay be set to X*2^(Y)−1 (or X*2^(Y)). Values X and Y can be transmittedthrough the TXOP duration field.

If the TXOP duration field transmitted in the HE-SIG A is K bits, n bits(first n bits) of the K bits may indicate the value X and m bits thereof(e.g., m bits at the end) may indicate the value Y. The n bits may be nMSBs or n LSBs and the m bits may be m LSBs or m MSBs. The values K, mand n can be set in various manners.

For example, it is assumed that K=6, n=3 and m=3. When X␣{1, 5, 10, 20,30, 40, 50, 60} and Y□{0˜7}, the TXOP duration may have a value in therange of 0 to 7,680 μs.

If a maximum of P ms (e.g., 5 ms) is indicated through the TXOP durationfield (e.g., in the HE-SIG A), an (X, Y) combination that minimizesX*2^(Y)−1, from among (X, Y) combinations satisfying X*2^(Y)−1≥P ms(e.g., 5 ms), may be used to indicate a maximum TXOP duration value andother (X, Y) combinations may not be used.

This example is one of methods of indicating the TXOP duration throughX*Z^(Y)−1 and thus X, Y and/or Z may be changed in various manners.

Example 4

According to an embodiment, the TXOP duration may be set in other unitsinstead of 1 microsecond (μs) (e.g., larger units or the unit of asymbol). For example, larger units such as 4 μs, 8 μs, 10 μs, 16 μs, 32μs, 50 μs, 64 μs, 100 μs, 128 μs, 256 μs, 500 μs, 512 μs, 1024 μs, . . .can be used. In this case, the TXOP duration value may be determined as“unit (e.g., 64 μs)*value of TXOP duration field”. For example, in thecase of 32 μs, TXOP Duration (1)=32 μs, TXOP Duration (2)=64 μs, TXOPDuration (3)=96 μs, . . . .

Meanwhile, it is desirable that the TXOP duration have a maximum valueof 8 ms. Accordingly, in a case where a single unit is used, thefollowing TXOP duration field options may be considered.

-   -   Option 1: A unit of 32 μs is used an 8-bit TXOP duration field        is defined. Here, the maximum TXOP duration value can be 8,192        μs.    -   Option 2: A unit of 64 μs is used and a 7-bit TXOP duration        field is defined. Here, the maximum TXOP duration value can be        8,192 μs.

If the TXOP field is set to more than 8 bits (e.g., 9 to 11 bits), thefollowing TXOP duration field structures may be used.

-   -   Option 1-1: 16 μs unit, ˜32 ms, 11 bits    -   Option 1-2: 16 μs unit, ˜16 ms, 10 bits    -   Option 1-3: 16 μs unit, ˜8 ms, 9 bits    -   Option 2-1: 32 μs unit, ˜32 ms, 10 bits    -   Option 2-2: 32 μs unit, ˜16 ms, 9 bits    -   Option 3-1: 64 μs unit, ˜16 ms, 9 bits

In addition, a combination of one or more units (e.g., (16 μs, 512 μs)or (8 μs, 128 μs), etc.) may be used. Or, 1× symbol or 4× symbol unitmay be used instead of μs, or the TXOP duration may be indicated by N*1×symbols or N*4× symbols (N being a natural number).

Table 1 illustrates a TXOP duration indicated by 4× symbols.

TABLE 1 TXOP duration field Actual value (units: 4x symbol) 0 0 1 1 4xsymbol (i.e., 16 μs) 2 2 4x symbols (i.e., 32 μs) 3 3 4x symbols (i.e.,48 μs) 4 4 4x symbols (i.e., 64 μs) 5 5 4x symbols (i.e., 80 μs) . . . .. .

The TXOP duration may be indicated by a combination of one of examples1/2/3 and example 4.

Example 5

According to an embodiment, the TXOP duration field may have apredefined value. A table in which values (e.g., a TXOP duration index)set to the TXOP duration field and actual TXOP duration values aremapped may be predefined. Table 2 illustrates TXOP duration indices.

TABLE 2 TXOP duration field Actual value (units: μs) 0 A 1 B 2 C 3 D 4 E5 F . . . . . .

According to an embodiment, part of the range of the TXOP duration maybe represented/configured as a first function form and another part ofthe range may be represented/configured as a second function form. Forexample, TXOP duration values may be set such that TXOP duration valuesincrease in an exponential function to a specific value and TXOPduration values following the specific value increase in a uniformdistribution function.

Table 3 illustrates a case in which the TXOP duration field is set to 4bits. Referring to Table 3, the TXOP duration exponentially increases inthe range of 32 μs to 512 μs (or 1,024 μs) and increases by 512 μs(approximately 0.5 ms) after 512 μs (or 1,024 μs).

TABLE 3 TXOP Duration field Actual value (units: us)  0 0  1 32  2 64  3128  4 256  5 512  6 1624  7 1536  8 2648  9 2560 10 3072 11 3584 124696 13 4608 14 5120 15 5632

Table 4 illustrates a case in which the TXOP duration field is set to 5bits. Referring to Table 4, the TXOP duration exponentially increases inthe range of 32 μs to 256 μs (or 512 μs) and increases by 256 μs(approximately 0.25 ms) after 256 μs (or 512 μs).

TABLE 4 TXOP Duration field Actual value (units: us)  0 0  1 32  2 64  3128  4 256  5 512  6 768  7 1024  8 1280  9 1536 10 1792 11 2048 12 230413 2560 14 2816 15 3072 16 3382 17 3584 18 3846 19 4096 20 4352 21 460822 4864 23 5120 24 5376 25 5632 26 5888 27 Reserved 28 Reserved 29Reserved 30 Reserved 31 Reserved

The following table 5 shows various examples of TXOP values indicated byindices of a 4-bit TXOP duration field. Cases A to H of Table 5 canrepresent different examples.

TABLE 5 Value (us) TXOP case case case case case case case case index AB C D E F G H 0 (b0000) 0 16 0 32 0 0 0 0 1 (b0001) 16 32 32 64 8 16 816 2 (b0010) 32 48 64 96 16 32 16 32 3 (b0011) 48 64 96 128 32 64 32 644 (b0100) 64 80 128 160 64 128 64 128 5 (b0101) 80 96 160 192 128 256128 256 6 (b0110) 96 112 192 224 256 512 256 512 7 (b0111) 112 128 224256 512 1024 512 1024 8 (b1000) 512 512 512 512 1024 1536 1024 2048 9(b1001) 1024 1024 1024 1024 1536 2048 2048 3072 10 (b1010) 1536 15361536 1536 2048 2560 3072 4096 11 (b1011) 2048 2048 2048 2048 2560 30724096 5120 12 (b1100) 2560 2560 2560 2560 3072 3584 5120 6144 13 (b1101)3072 3072 3072 3072 3584 4096 6144 7168 14 (b1110) 3584 3584 3584 35844096 4608 7168 8192 15 (b1111) 4096 4096 4096 4096 4608 5120 8192 9216

(i) In case A, the TXOP duration value is determined as (16 μs*(thevalue of the remaining 3 bits)) when the MSB of the indices is 0. TheTXOP duration value is determined as (512 μs*(the value of the remaining3 bits)+1) when the MSB of the indices is 1.

(ii) In case B, the TXOP duration value is determined as (16 μs*(thevalue of the remaining 3 bits)+1) when the MSB of the indices is 0. TheTXOP duration value is determined as (512 μs*(the value of the remaining3 bits)+1) when the MSB of the indices is 1.

(iii) In case C, the TXOP duration value is determined as (32 μs*(thevalue of the remaining 3 bits)) when the MSB of the indices is 0. TheTXOP duration value is determined as (512 μs*(the value of the remaining3 bits)+1) when the MSB of the indices is 1.

(iv) In case D, the TXOP duration value is determined as (32 μs*(thevalue of the remaining 3 bits)+1) when the MSB of the indices is 0. TheTXOP duration value is determined as (512 μs*(the value of the remaining3 bits)+1) when the MSB of the indices is 1.

(v) In cases E to H, the TXOP duration value can be understood as in (i)to (iv). For example, the MSB of the indices can be understood as ascaling factor, granularity or duration unit of the TXOP duration (referto embodiments which will be described below).

Example 6

According to an embodiment, the TXOP duration can be set through anX-bit scaling factor and a Y-bit duration value. For example, the TXOPduration can be set on the basis of Scaling factor (X bits)*Duration (Ybits). Specifically, TXOP duration=Scaling factor (X bits)*Duration (Ybits). Otherwise, TXOP duration=Scaling factor (X bits)*Duration (Ybits)+a, a being a predetermined constant (e.g., a=1).

The size of the TXOP duration field can be set to X+Y bits.

For example, the unit of the duration value can be set to one of 1 μs, 4μs and 16 μs according to the scaling factor. The length of the Y bitscan be set to various values.

Scaling factor index 0 of the X bits can indicate actual scalingfactor=0. Case A and case B of Table 6 show examples of a 2-bit scalingfactor.

TABLE 6 Value Scaling factor field Case A Case B 0 0 0 1 4 1 2 16 10 332 100

Table 7 shows examples of a 3-bit scaling factor.

TABLE 7 Scaling factor field Value 0 0 1 1 2 4 3 16 4 32 5 64 6 128 7256

The duration value may be represented in the form of 2^(Y).

Table 8 illustrates a scaling factor set to 1 bit. Referring to Table 8,scaling factor=0 can indicate 16 μs and scaling factor=1 can indicate512 μs. For example, the TXOP duration can be set to 16*Duration (μs)when scaling factor=0 and set to 512*Duration (μs) when scalingfactor=1. The unit of Duration is assumed to be 1 μs for convenience.

TABLE 8 Scaling factor field Value (us) 0 16 1 512

Table 9 shows examples of a 5-bit TXOP duration field. In Table 9, it isassumed that the scaling factor is set to the MSB as in Table 8.Accordingly, the remaining 4 bits other than the MSB used as the scalingfactor in the 5-bit TXOP Duration field are used as a duration fieldvalue, and thus the 4-bit duration field value can be one of 0 to 15.

Case A of Table 9 shows an example in which the actual TXOP durationvalue is set to (Scaling factor value*Duration field value) (e.g., avalue of 4 bits other than the MSB).

Case B of Table 9 shows an example in which the actual TXOP durationvalue is set to (Scaling factor value*(Duration field value+1)).

In Case C of Table 9, the actual TXOP duration value is set to (Scalingfactor value (16 μs)*Duration field value) when scaling factor=0 (e.g.,the unit of the scaling factor value is 16 μs) and set to (Scalingfactor value (512 μs)*(Duration field value+1)) when scaling factor=1(e.g., the unit of the scaling factor value is 512 μs).

TABLE 9 TXOP Duration field Actual value (units: us) (MSB: Scalingfactor) Case A Case B Case C  0 0 16 0  1 16 32 16  2 32 48 32  3 48 6448  4 64 80 64  5 80 96 80  6 96 112 96  7 112 128 112  8 128 144 128  9144 160 144 10 160 176 160 11 176 192 176 12 192 208 192 13 208 224 20814 224 240 224 15 240 256 240 16 0 512 512 17 512 1024 1024 18 1024 15361536 19 1536 2048 2048 20 2048 2560 2560 21 2560 3072 3072 22 3072 35843584 23 3584 4096 4096 24 4096 4608 4608 25 4608 5120 5120 26 5120 56325632 27 5632 6144 6144 28 6144 6656 6656 29 6656 7168 7168 30 7168 76807680 31 7680 8192 8192

Table 10 shows other examples of the 1-bit scaling factor. Referring toTable 10, scaling factor=0 can indicate 32 μs and scaling factor=1 canindicate 512 μs.

TABLE 10 Scaling factor field Value (us) 0 32 1 512

Table 11 shows other examples of the 5-bit TXOP duration field. In Table11, it is assumed that the scaling factor is set to the MSB as in Table9. Accordingly, the remaining 4 bits other than the MSB used as thescaling factor in the 5-bit TXOP Duration field are used as a durationfield value, and thus the 4-bit duration field value can be one of 0 to15. Referring to Table 11, the actual TXOP duration value can be set to32*Duration (μs) when scaling factor=0 and set to 512*(Duration+1) (μs)when scaling factor=1. The unit of Duration is assumed to be 1 μs forconvenience.

TABLE 11 TXOP Duration field (MSB: Scaling factor) Actual value (units:us)  0 0  1 32  2 64  3 96  4 128  5 160  6 192  7 224  8 256  9 288 10320 11 352 12 384 13 416 14 448 15 480 16 512 17 1024 18 1536 19 2048 202560 21 3072 22 3584 23 4096 24 4608 25 5120 26 5632 27 6144 28 6656 297168 30 7680 31 8192

Table 12 shows other examples of the 1-bit scaling factor. Referring toFIG. 12, scaling factor=0 can indicate 32 μs and scaling factor=1 canindicate 1,024 μs.

TABLE 12 Scaling factor field Value (us) 0 32 1 1024

Table 13 shows other examples of the 5-bit TXOP duration field. In Table13, it is assumed that the scaling factor as in Table 12 is set to theMSB. Accordingly, the remaining 4 bits other than the MSB used as thescaling factor in the 5-bit TXOP Duration field are used as a durationfield value, and thus the 4-bit duration field value can be one of 0 to15. Referring to Table 13, the actual TXOP duration value can be set to32*Duration (μs) when scaling factor=0 and set to 1,024*(Duration+1)(μs) when scaling factor=1. The unit of Duration is assumed to be 1 μsfor convenience.

TABLE 13 TXOP Duration field (MSB: Scaling factor) Actual value (units:us)  0 32  1 64  2 96  3 128  4 160  5 192  6 224  7 256  8 288  9 32010 352 11 384 12 416 13 448 14 480 15 512 16 1024 17 2048 18 3072 194096 20 5120 21 6144 22 7168 23 8192 24 9216 25 10240 26 11264 27 1228828 13312 29 14336 30 15360 31 16384

Table 14 shows examples of a 6-bit TXOP duration field. In Table 14, itis assumed that a 1-bit scaling factor is set to the MSB as in Table 8.Accordingly, the remaining 5 bits other than the MSB used as the scalingfactor in the 6-bit TXOP Duration field are used as a duration fieldvalue, and thus the 5-bit duration field value can be one of 0 to 31.Referring to Table 14, the actual TXOP duration value can be set to16*Duration (μs) when scaling factor=0 and set to 512*(Duration+1) (us)when scaling factor=1. The unit of Duration is assumed to be 1 μs forconvenience.

TABLE 14 Actual value TXOP Duration field (units: us)  0 0  1 16  2 32 3 48  4 64  5 80  6 96  7 112  8 128  9 144 10 160 11 176 12 192 13 20814 224 15 240 16 256 17 272 18 288 19 304 20 320 21 336 22 352 23 368 24384 25 400 26 416 27 432 28 448 29 464 30 480 31 496 32 512 33 1024 341536 35 2048 36 2560 37 3072 38 3584 39 4096 40 4608 41 5120 42 5632 436144 44 6656 45 7168 46 7680 47 8192 48 8704 49 9216 50 9728 51 10240 5210752 53 11264 54 11776 55 12288 56 12800 57 13312 58 13824 59 14336 6014848 61 15360 62 15872 63 16384

Table 15 shows other examples of the 6-bit TXOP duration field. In Table15, it is assumed that the 1-bit scaling factor is set to the MSB as inTable 8. Accordingly, the remaining 5 bits other than the MSB used asthe scaling factor in the 6-bit TXOP Duration field are used as aduration field value, and thus the 5-bit duration field value can be oneof 0 to 31. Referring to Table 15, the actual TXOP duration value can beset to 16*(Duration+1) (μs) when scaling factor=0 and set to512*(Duration+1) (μs) when scaling factor=1. The unit of Duration isassumed to be 1 μs for convenience.

TABLE 15 Actual value TXOP Duration field (units: us)  0 16  1 32  2 48 3 64  4 80  5 96  6 112  7 128  8 144  9 160 10 176 11 192 12 208 13224 14 240 15 256 16 272 17 288 18 304 19 320 20 336 21 352 22 368 23384 24 400 25 416 26 432 27 448 28 464 29 480 30 496 31 512 32 512 331024 34 1536 35 2048 36 2560 37 3072 38 3584 39 4096 40 4608 41 5120 425632 43 6144 44 6656 45 7168 46 7680 47 8192 48 8704 49 9216 50 9728 5110240 52 10752 53 11264 54 11776 55 12288 56 12800 57 13312 58 13824 5914336 60 14848 61 15360 62 15872 63 16384

Table 16 shows other examples of the 6-bit TXOP duration field. In Table16, it is assumed that the 1-bit scaling factor is set to the MSB as inTable 10. Accordingly, the remaining 5 bits other than the MSB used asthe scaling factor in the 6-bit TXOP Duration field are used as aduration field value, and thus the 5-bit duration field value can be oneof 0 to 31. Referring to Table 16, the actual TXOP duration value can beset to 32*Duration (μs) when scaling factor=0 and set to512*(Duration+2) (μs) when scaling factor=1. The unit of Duration isassumed to be 1 μs for convenience.

TABLE 16 Actual value TXOP Duration field (units: us)  0 0  1 32  2 64 3 96  4 128  5 160  6 192  7 224  8 256  9 288 10 320 11 352 12 384 13416 14 448 15 480 16 512 17 544 18 576 19 608 20 640 21 672 22 704 23736 24 768 25 800 26 832 27 864 28 896 29 928 30 960 31 992 32 1024 331536 34 2048 35 2560 36 3072 37 3584 38 4096 39 4608 40 5120 41 5632 426144 43 6656 44 7168 45 7680 46 8192 47 8704 48 9216 49 9728 50 10240 5110752 52 11264 53 11776 54 12288 55 12800 56 13312 57 13824 58 14336 5914848 60 15360 61 15872 62 16384 63 16896

Table 17 shows other examples of the 6-bit TXOP duration field. In Table17, it is assumed that the 1-bit scaling factor is set to the MSB as inTable 12. Accordingly, the remaining 5 bits other than the MSB used asthe scaling factor in the 6-bit TXOP Duration field are used as aduration field value, and thus the 5-bit duration field value can be oneof 0 to 31. Referring to Table 17, the actual TXOP duration value can beset to 32*Duration (μs) when scaling factor=0 and set to1,024*(Duration+1) (μs) when scaling factor=1. The unit of Duration isassumed to be 1 μs for convenience.

TABLE 17 Actual value TXOP Duration field (unit: us)  0 0  1 32  2 64  396  4 128  5 160  6 192  7 224  8 256  9 288 10 320 11 352 12 384 13 41614 448 15 480 16 512 17 544 18 576 19 608 20 640 21 672 22 704 23 736 24768 25 800 26 832 27 864 28 896 29 928 30 960 31 992 32 1024 33 1536 342048 35 2560 36 3072 37 3584 38 4096 39 4608 40 5120 41 5632 42 6144 436656 44 7168 45 7680 46 8192 47 8704 48 9216 49 9728 50 10240 51 1075252 11264 53 11776 54 12288 55 12800 56 13312 57 13824 58 14336 59 1484860 15360 61 15872 62 16384 63 16896

Table 18 shows other examples of the 6-bit TXOP duration field.Referring to Table 18, the actual TXOP duration value increases in unitsof 32 μs until 512 μs and increases in units of 512 μs after 512 μs.

TABLE 18 Actual value TXOP Duration field (unit: us)  0 0  1 32  2 64  396  4 128  5 160  6 192  7 224  8 256  9 288 10 320 11 352 12 384 13 41614 448 15 480 16 512 17 544 18 576 19 608 20 640 21 672 22 704 23 736 24768 25 800 26 832 27 864 28 896 29 928 30 960 31 992 32 1024 33 2048 343072 35 4096 36 5120 37 6144 38 7168 39 8192 40 9216 41 10240 42 1126443 12288 44 13312 45 14336 46 15360 47 16384 48 17408 49 18432 50 1945651 20480 52 21504 53 22528 54 23552 55 24576 56 25600 57 26624 58 2764859 28672 60 29696 61 30720 62 31744 63 32768

Example 7

According to an embodiment, the TXOP duration may be indicated throughX-bit scaling factor, Y-bit duration value and Z-bit duration unitinformation. The TXOP duration may be “Scaling factor (X bits)*(Duration(Y bits) μs*Duration unit (Z bits) μs).” The size of the TXOP durationfield may be set to (X+Y+Z) bits.

The Z-bit duration unit represents the unit of transmitted durationinformation. For example, when the Z bit is 1 bit, 0 can indicate theunit of 4 μs and 1 can indicate the unit of 16 μs. However, the presentinvention is not limited thereto.

Example 8

When the TXOP duration field is included in the HE-SIG A of the HE PPDU,the length of the TXOP duration, granularity and the like indicated bythe TXOP duration field need to be defined. For example, (1) size, (2)maximum value and (3) granularity need to be determined in considerationof the capacity of the HE-SIG A and the granularity of the TXOPduration. The granularity may be represented as a scaling (or scalingfactor) or a TXOP duration unit.

(1) Size of TXOP Duration Field

As to the capacity of the HE-SIG A, 13 remaining bits (e.g., bits thatare available since they are not defined for other purposes) in the caseof the HE SU PPDU format, 14 remaining bits are present in the case ofthe HE MU PPDU, and more than 14 remaining bits are present in the caseof the HE trigger-based PPDU.

As fields, sizes of which are not currently determined in the HE-SIG Afield, for example, BW (2 bits or more), spatial reuse and TXOP durationfields may be exemplified in the HE-MU PPDU format.

As other HE-SIG A fields under discussion, there are a 1-bit reservedfield similarly to the legacy system and 1-bit STBC in the case of theHE MU PPDU format.

Accordingly, the length of the TXOP duration field can be limited to aspecific size (e.g., 5 to 7 bits) in consideration of other fields ofthe HE-SIG A.

Furthermore, considering such size restriction, it is desirable that theTXOP duration field have a larger granularity than the MAC duration.That is, the TXOP duration field can have a larger granularity than theMAC duration although it is set to be smaller than the MAC duration.

(2) Maximum Value of TXOP Duration

As described above, the MAC duration field (e.g., 15 bits, unit of 1 μs)can cover up to approximately 32 ms. Although a TXOP limit isapproximately 4 ms in a default EDCA parameter set, an AP can set anEDCA parameter set through a beacon.

The AP may set the TXOP duration to be longer than 4 ms by the TXOPduration field (e.g., 8 or 16 ms). Particularly, the AP needs to set along TXOP duration in an MU TXOP procedure or cascade structure.

In LAA (Licensed Assisted Access) for using unlicensed bands in acellular system (e.g., 3GPP), a maximum TXOP is defined as 8 ms andWi-Fi requires a very long TXOP (e.g., up to 10 ms) for soundingpackets. According to European LBT (Listen Before Talk) requirements, amaximum channel occupation time can be 10 ms. According to LTE-U that isan LTE system operating in unlicensed bands, a maximum on-state durationis 20 ms.

Considering such design elements, it is desirable that a maximum TXOPduration size that can be indicated by the HE-SIG A field be 8 ms (or 16ms), for example.

(3) Granularity of TXOP Duration

When one relatively small granularity (e.g., 1 μs, 16 μs or the like) isused, the TXOP duration field requires a lot of bits (e.g., 8 to 15bits). Table 19 illustrates the number of bits of the TXOP duration andmaximum TXOP duration values, which are required when a singlegranularity is used.

TABLE 19 granularity (us) max TXOP duration (ms) number of bits Case 1 132 15 Case 2 16 14 Case 3 8 13 Case 4 16 32 11 Case 5 16 10 Case 6 8 9Case 7 32 32 10 Case 8 16 9 Case 9 8 8

Conversely, when only one relatively large granularity is used, anover-protection problem is frequently generated in STAs (e.g., thirdparty STAs) and thus channel use efficiency may decrease (e.g., a NAV isset to an unnecessarily large TXOP duration value).

FIG. 23 illustrates setting of a TXOP duration with a small granularityand setting of a TXOP duration with a large granularity. FIG. 23(a)shows TXOP duration setting for DL transmission and illustrates a casein which STAs transmit UL MU BA in response to a DL MU PPDU transmittedfrom an AP. FIG. 23(b) shows TXOP duration setting for UL transmissionand illustrates a case in which STAs transmit UL MU frames on the basisof a trigger frame transmitted from an AP and the AP transmits DL MU BA.Referring to FIGS. 23(a) and 23(b), the size of an error between a MACduration and a TXOP duration set by the TXOP duration field of theHE-SIG A is relatively small when the small granularity is used andrelatively large when the large granularity is used. In this way, use ofa large granularity may cause over-protection beyond actually requiredTXOP.

Meanwhile, from among relatively small packets (e.g., ACK, BA, MU BA,etc.), ACK or BA is positioned in the last frame of a TXOP. Durations ofACK, BA and/or MU BA depend on their data rates. For example, theduration of UL MU BA is 422.4 μs at a low data rate (e.g., MCS0, 26tones) (refer to FIG. 24).

FIG. 24 illustrates allocation of a UL OFDMA BA frame in MCS0.

The preamble of the UL OFDMA BA frame has a duration of 48 μs andincludes a legacy preamble and an HE preamble. The legacy preamble is 20μs and may include L-STF (8 μs), L-LTF (8 μs) and L-SIG (4 μs). The HEpreamble is 28 μs and may include RL (repetition legacy)-SIG (4 μs),HE-SIG A (8 μs), HE-STF (8 μs) and HE-LTF (8 μs).

The MAC frame of compressed BA may be set to 39 octets, that is, 312bits. Specifically, the MAC frame of compressed BA corresponds toservice field (2 octets)+MPDU delimiter (4 octets)+MAC header (16octets)+BA control (2 octets)+BA information (10 octets)+FCS (4octets)+tail (1 octet)=39 octets. The symbol length thereof is 12.8+1.6CP=14.4 μs. Accordingly, the MAC frame of compressed BA becomes 374.4 μswhen MCS0 and 26 tones are used.

Accordingly, when MCS0 and 26 tones are used, the duration of UL MU BAis set to 422.4 μs corresponding to the sum of 48 μs for the preambleand 374.4 μs for the MAC frame.

It may be more efficient to use a small granularity (e.g., less than 32μs) for at least part of ACK, BA and/or MU BA to solve over-protectionby third party STAs.

Accordingly, the TXOP duration needs to support small packets having asmall granularity (e.g., 16 or 32 μs). As a method for supporting suchsmall-capacity packets, a method of using multiple granularities (e.g.,small and large granularities) for TXOP may be considered.

According to an embodiment of the present invention, multiple units(e.g., multi-granularity) can be used for the TXOP duration. Althoughthe number of multiple units may be 2 (or 4), the number of multipleunits is not limited thereto. If the number of units is 2, respectiveunits may be referred to as a small unit and a large unit forconvenience. The actual sizes of the small unit and the large unit maydepend on the size of the TXOP duration field (e.g., 5, 6 or 7 bits).For example, the small unit can be used to indicate a duration of lessthan 512 μs and the large unit can be used to indicate a duration in therange of 512 μs to the maximum TXOP duration value (e.g., approximately8 ms). For example, the small unit can be used for the above-describedUL MU BA (e.g., having a duration of approximately 400 μs) of the lowestdata rate.

Table 20 illustrates a small unit and a large unit depending on a TXOPduration field size.

TABLE 20 TXOP Max duration value of Field TXOP Small Large size (bits)duration (us) unit (us) unit (us) Option 1-1 5 8192 16 512 Option 1-2 32Option 2-1 6 8192 (or 8448) 16 256 Option 2-2 16384 512 Option 3-0 78192 (or 8576) 8 128 Option 3-1 8832 4/8/16 256 Option 3-2 8704 8 Option3-3 12616 16

(i) Example of Option 1-1 of Table 20: 5-Bit Field Size, 2 Units (16 usand 512 us)

Table 21 illustrates TXOP duration values depending on TXOP durationfield values (e.g., TXOP indices) in a case in which the TXOP durationfield is 5 bits (e.g., B0˜B4), small unit=16 μs and large unit=512 μs(option 1-1 of Table 20).

TABLE 21 TXOP B0 B1~B4 duration range Unit TXOP duration value 00000~1111  0 us~240 us  16 us (16 * value of (B1~B4)) us 1 0000~1111 512us~8192 us 512 us (512 + 512 * value of (B1~B4) us

Referring to Table 21, B0 indicates the unit (or granularity) of aduration. For example, B0=0 indicates a small unit of 16 μs and B0=1indicates a large unit of 512 μs. Accordingly, an STA can calculate aTXOP duration value on the basis of values of B0 to B4 of the TXOPduration field of the HE-SIG A field. For example, TXOP durationvalue=(16*value of (B1˜B4)) μs when B0=0 and TXOP durationvalue=(512+512*value of (B1˜B4)) μs when B0=1.

(ii) Example of Option 1-2 of Table 20: 5-Bit Field Size, 2 Units (32 usand 512 us)

Table 22 illustrates TXOP duration values depending on TXOP durationfield values (e.g., TXOP indices) in a case in which the TXOP durationfield is 5 bits (e.g., B0˜B4), small unit=32 μs and large unit=512 μs(option 1-2 of Table 20).

TABLE 22 TXOP B0 B1~B4 duration range Unit TXOP duration value 00000~1111  0 us~480 us  32 us (32 * value of (B1~B4)) us 1 0000~1111 512us~8192 us 512 us (512 + 512 * value of (B1~B4) us

Referring to Table 22, B0 indicates the unit (or granularity) of aduration. For example, B0=0 indicates a small unit of 32 μs and B0=1indicates a large unit of 512 μs. Accordingly, an STA can calculate aTXOP duration value on the basis of values of B0 to B4 of the TXOPduration field of the HE-SIG A field. For example, TXOP durationvalue=(32*value of (B1˜B4)) μs when B0=0 and TXOP durationvalue=(512+512*value of (B1˜B4)) μs when B0=1.

Meanwhile, the STA may acquire a TXOP duration value from a predefinedlookup table. For example, the STA may use a lookup table such as Table23 instead of calculating a TXOP duration value every time. Table 23shows results calculated according to the above-described TXOP durationvalue calculation method.

TABLE 23 TXOP duration TXOP Index Value (us)  0   0  1  32  2  64  3  96 4  128  5  160  6  192  7  224  8  256  9  288 10  320 11  352 12  38413  416 14  448 15  480 16  512 17 1024 18 1536 19 2048 20 2560 21 307222 3584 23 4096 24 4608 25 5120 26 5632 27 6144 28 6656 29 7168 30 768031 8192

TXOP indices in the left column of Table 23 correspond to B0=0 and TXOPindices in the right column correspond to B0=1. For example, the unit of32 μs is applied to TXOP indices 0 and 1 and the unit of 512 μs isapplied to TXOP indices 16 and 17.

(iii) Example of Option 2-1 of Table 20: 6-Bit Field Size, 2 Units (16us and 256 us)

Table 24 illustrates TXOP duration values depending on TXOP durationfield values (e.g., TXOP indices) in a case in which the TXOP durationfield is 6 bits (e.g., B0˜B5), small unit=16 μs and large unit=256 μs(option 2-1 of Table 20).

TABLE 24 TXOP B0 B1~B5 duration range Unit TXOP duration value 000000~11111  0 us~496 us  16 us (16 * value of (B1~B5)) us 1 00000~11111512 us~8448 us 256 us (512 + 256 * value of (B1~B5) us

Referring to Table 24, B0 indicates the unit (or granularity) of aduration. For example, B0=0 indicates a small unit of 16 μs and B0=1indicates a large unit of 256 μs. Accordingly, an STA can calculate aTXOP duration value on the basis of values of B0 to B5 of the TXOPduration field of the HE-SIG A field. For example, TXOP durationvalue=(16*value of (B1˜B5)) μs when B0=0 and TXOP durationvalue=(512+256*value of (B1˜B5)) μs when B0=1.

Meanwhile, the STA may acquire a TXOP duration value from a predefinedlookup table. For example, the STA may use a lookup table such as Table25 instead of calculating a TXOP duration value every time. Table 25shows results calculated according to the above-described TXOP durationvalue calculation method.

TABLE 25 TXOP duration TXOP Index Value (us) 0 0 1 16 2 32 3 48 4 64 580 6 96 7 112 8 128 9 144 10 160 11 176 12 192 13 208 14 224 15 240 16256 17 272 18 288 19 304 20 320 21 336 22 352 23 368 24 384 25 400 26416 27 432 28 448 29 464 30 480 31 496 32 512 33 768 34 1024 35 1280 361536 37 1792 38 2048 39 2304 40 2560 41 2816 42 3072 43 3328 44 3584 453840 46 4096 47 4352 48 4608 49 4864 50 5120 51 5376 52 5632 53 5888 546144 55 6400 56 6656 57 6912 58 7168 59 7424 60 7680 61 7936 62 8192 638448

(iv) Example of Option 2-2 of Table 20: 6-Bit Field Size, 2 Units (16 usand 512 us)

Table 26 illustrates TXOP duration values depending on TXOP durationfield values (e.g., TXOP indices) in a case in which the TXOP durationfield is 6 bits (e.g., B0˜B5), small unit=16 μs and large unit=512 μs(option 2-2 of Table 20).

TABLE 26 TXOP B0 B1~B5 duration range Unit TXOP duration value 000000~11111  0 us~496 us   16 us (16 * value of (B1~B5)) us 100000~11111 512 us~16348 us 512 us (512 + 512 * value of (B1~B5) us

Referring to Table 26, B0 indicates the unit (or granularity) of aduration. For example, B0=0 indicates a small unit of 16 μs and B0=1indicates a large unit of 512 μs. Accordingly, an STA can calculate aTXOP duration value on the basis of values of B0 to B5 of the TXOPduration field of the HE-SIG A field. For example, TXOP durationvalue=(16*value of (B1˜B5)) μs when B0=0 and TXOP durationvalue=(512+512*value of (B1˜B5)) μs when B0=1.

Meanwhile, the STA may acquire a TXOP duration value from a predefinedlookup table. A lookup table corresponding to Table 26 is omitted forconvenience.

(v) Example of Option 3-0 of Table 20: 7-Bit Field Size, 2 Units (8 usand 128 us

Table 27 illustrates TXOP duration values depending on TXOP durationfield values (e.g., TXOP indices) in a case in which the TXOP durationfield is 7 bits (e.g., B0˜B6), small unit=8 μs and large unit=128 μs(option 3-0 of Table 20).

TABLE 27 TXOP B0 B1~B5 duration range Unit TXOP duration value 0000000~111111  0 us~504 us   8 us (8 * value of (B1~B6)) us 1000000~111111 512 us~8576 us 128 us (512 + 128 * value of (B1~B6) us

Referring to Table 27, B0 indicates the unit (or granularity) of aduration. For example, B0=0 indicates a small unit of 8 μs and B0=1indicates a large unit of 128 μs. Accordingly, an STA can calculate aTXOP duration value on the basis of values of B0 to B6 of the TXOPduration field of the HE-SIG A field. For example, TXOP durationvalue=(8*value of (B1˜B6)) μs when B0=0 and TXOP durationvalue=(512+128*value of (B1˜B6)) μs when B0=1.

Meanwhile, the STA may acquire a TXOP duration value from a predefinedlookup table. Table 28 is a lookup table corresponding to Table 27.

TABLE 28 TXOP Index TXOP duration Value (us) 0 0 1 8 2 16 3 24 4 32 5 406 48 7 56 8 64 9 72 10 80 11 88 12 96 13 104 14 112 15 120 16 128 17 13618 144 19 152 20 160 21 168 22 176 23 184 24 192 25 200 26 208 27 216 28224 29 232 30 240 31 248 32 256 33 264 34 272 35 280 36 288 37 296 38304 39 312 40 320 41 328 42 336 43 344 44 352 45 360 46 368 47 376 48384 49 392 50 400 51 408 52 416 53 424 54 432 55 440 56 448 57 456 58464 59 472 60 480 61 488 62 496 63 504 64 512 65 640 66 768 67 896 681024 69 1152 70 1280 71 1408 72 1536 73 1664 74 1792 75 1920 76 2048 772176 78 2304 79 2432 80 2560 81 2688 82 2816 83 2944 84 3072 85 3200 863328 87 3456 88 3584 89 3712 90 3840 91 3968 92 4096 93 4224 94 4352 954480 96 4608 97 4736 98 4864 99 4992 100 5120 101 5248 102 5376 103 5504104 5632 105 5760 106 5888 107 6016 108 6144 109 6272 110 6400 111 6528112 6656 113 6784 114 6912 115 7040 116 7168 117 7296 118 7424 119 7552120 7680 121 7808 122 7936 123 8064 124 8192 125 8320 126 8448 127 8576

TXOP indices in the left two columns of Table 28 correspond to B0=0 andTXOP indices in the right two columns correspond to B0=1. For example,the unit of 8 μs is applied to TXOP indices 0 and 32 and the unit of 128μs is applied to TXOP indices 64 and 96.

(vi) Example of Option 3-1 of Table 20: 7-Bit Field Size, 4 Units (4 us,8 us, 16 us and 256 us)

Table 29 illustrates TXOP duration values depending on TXOP durationfield values (e.g., TXOP indices) in a case in which the TXOP durationfield is 7 bits (e.g., B0˜B6) and a total of 4 duration units of 4 μs, 8μs, 16 μs and 256 μs (option 3-1 of Table 20). For example, 4 μs, 8 μsand 16 μs may correspond to small units and 256 μs may correspond to alarge unit.

TABLE 29 TXOP B0B1 B2~B6 duration range Unit TXOP duration value 0000000~11111   0 us~124 us  4 us (4 * value of (B2~B6)) us 01 00000~11111128 us~376 us  8 us (128 + 8 * value of (B2~B6)) us 10 00000~11111 384us~880 us  16 us (384 + 16 * value of (B2~B6)) us 11 00000~11111  896us~8832 us 256 us (896 + 256 * value of (B2~B6)) us

Referring to FIG. 29, B0B1 indicates one of 4 duration units (orgranularities). For example, B0B1=00 indicates 4 μs, B0B1=01 indicates 8μs, B0B1=10 indicates 16 μs and B0B1=11 indicates 256 μs.

Accordingly, an STA can calculate a TXOP duration value on the basis ofvalues B0 to B6 of the TXOP duration field of the HE-SIG A field. Forexample, TXOP duration value=(4*value of (B2˜B6)) μs when B0B1=00, TXOPduration value=(128+8*value of (B2˜B6)) μs when B0B1=01, TXOP durationvalue=(384+16*value of (B2˜B6)) μs when B0B1=10 and TXOP durationvalue=(896+256*value of (B2˜B6)) us when B0B1=11.

Meanwhile, the STA may acquire a TXOP duration value from a predefinedlookup table. A lookup table corresponding to Table 29 is omitted forconvenience.

(vii) Example of Option 3-2 of Table 20: 7-Bit Field Size, 2 Units (8 usand 256 us)

Table 30 illustrates TXOP duration values depending on TXOP durationfield values (e.g., TXOP indices) in a case in which the TXOP durationfield is 7 bits (e.g., B0˜B6), small unit=8 μs and large unit=256 μs(option 3-2 of Table 20).

TABLE 30 B0B1 B2~B6 TXOP duration range Unit TXOP duration value 0000000~11111   0 us~248 us  8 us (8 * value of (B2~B6)) us 01 00000~11111256 us~506 us (256 + 8 * value of (B2~B6)) us 10 00000~11111 512 us~760us (512 + 8 * value of (B2~B6)) us 11 00000~11111  768 us~8704 us 256 us(768 + 256 * value of (B2~B6)) us

Referring to FIG. 30, B0B1 indicates one of 2 duration units (orgranularities). In addition, B0B1 indicates duration values of B2˜B3(00000). For example, B0B1=00 indicates 8 μs and B2˜B3(00000)=0. B0B1=01indicates 8 μs and B2˜B3(00000)=256. B0B1=10 indicates 8 μs andB2˜B3(00000)=512. B0B1=11 indicates 256 μs and B2˜B3(00000)=768 μs.

Accordingly, an STA can calculate a TXOP duration value on the basis ofvalues B0 to B6 of the TXOP duration field of the HE-SIG A field. Forexample, TXOP duration value=(8*value of (B2˜B6)) μs when B0B1=00, TXOPduration value=(256+8*value of (B2˜B6)) μs when B0B1=01, TXOP durationvalue=(512+8*value of (B2˜B6)) μs when B0B1=10 and TXOP durationvalue=(768+256*value of (B2˜B6)) μs when B0B1=11.

Meanwhile, the STA may acquire a TXOP duration value from a predefinedlookup table. A lookup table corresponding to Table 30 is omitted forconvenience.

(viii) Example of Option 3-3 of Table 20: 7-Bit Field Size, 2 Units (16us and 256 us)

Table 31 illustrates TXOP duration values depending on TXOP durationfield values (e.g., TXOP indices) in a case in which the TXOP durationfield is 7 bits (e.g., B0˜B6), small unit=16 μs and large unit=256 μs(option 3-3 of Table 20).

TABLE 31 B0 B1~B6 TXOP duration range Unit TXOP duration value 0000000~111111   0 us~1008 us  16 us (16 * value of (B1~B6)) us 1000000~111111 1024 us~12616 us 256 us (1024 + 256 * value of (B1~B6)) us

Referring to Table 31, B0 indicates a duration unit (or granularity).For example, B0=0 indicates a small unit of 16 μs and B0=1 indicates alarge unit of 256 μs. Accordingly, an STA can calculate a TXOP durationvalue on the basis of values B0 to B6 of the TXOP duration field of theHE-SIG A field. For example, TXOP duration value=(16*value of (B1˜B6))μs when B0=0 and TXOP duration value=(1024+256*value of (B1˜B6)) μs whenB0=1.

Meanwhile, the STA may acquire a TXOP duration value from a predefinedlookup table. A lookup table corresponding to Table 31 is omitted forconvenience.

Table 32 shows throughput and gains with respect to the above-describedexamples. In Table 32, it is assumed that there are 32 BSSs, a maximumof 64 STAs are present per BSS and reuse factor=4. In addition, 20 MHzchannels on 5 GHz and 2Tx-2Rx are assumed. Furthermore, it is assumedthat a buffer state is a full buffer state, TXOP is 2 ms and RTS is inan off state. The left column of Table 32 represents a case in which theCF-END frame is not used and the right column of Table 32 represents acase in which the CF-END frame is used.

Referring to Table 32, the influence of most small units (e.g., up to 16μs) on performance is relatively small. For example, 8/16 μs havethroughput loss of 0.7%/1% compared to 1 μs.

When large units are used, use of the CF-END frame to truncate theremaining TXOP is more advantageous to improvement of system throughputand gain.

TABLE 32 CF-END off CF-END on unit (μs) Thpt (Mbps) Gain Thpt (Mbps)Gain 1024 247.648 −49.99% 464.871  −6.08% 512 331.483 −33.05% 466.109 −5.83% 256 394.352 −20.36% 465.743  −5.91% 128 439.931 −11.15% 464.396 −6.18% 64 466.632  −5.76% 465.582  −5.94% 32 481.309  −2.80% 479.736 −3.08% 16 489.537  −1.13% 488.327  −1.34% 8 491.776 −0.682% 491.174−0.769% 4 493.564 −0.321% 493.129 −0.375% 1 (original) 495.153    0.00%494.983    0.00%

(4) Determination of TXOP Duration Value

An STA (e.g., a TXOP holder/responder) transmitting frames needs todetermine and calculate a TXOP duration value that the STA intends tosignal through the TXOP duration field of the HE-SIG A. For example, theSTA can determine a TXOP duration value (e.g., a value indicated by theTXOP duration field of HE-SIG A) on the basis of the duration of a MACheader included in a frame that the STA transmits (e.g., the durationfield of the MAC header of MPDU).

FIG. 25 illustrates a method of setting a TXOP duration value accordingto an embodiment. In the present embodiment, it is assumed that MACduration value=D.

Referring to FIG. 25, an STA can set TXOP duration value=ceiling(D/granularity)*granularity. Here, ceiling (A) represents the smallestinteger from among integers equal to or greater than A. Accordingly, theTXOP duration value is set to be greater than MAC duration value D. Forexample, TXOP duration value=MAC duration value D is satisfied when theMAC duration value D is a multiple of the granularity and TXOP durationvalue >MAC duration value D is satisfied in other cases.

For example, when D=100 μs and granularity=16 μs, TXOP durationvalue=ceiling (100/16)*16=112 μs. When TXOP duration value=1,024 μs issignaled in the same way as option 2-2 of Table 20 (e.g., 6-bitduration, small unit=16 μs and large unit=512 μs), the TXOP durationfield value (TXOP index) is set to 7.

In another example, when D=900 μs and granularity=512 μs, TXOP durationvalue=ceiling (900/512)*512=1,024 μs. When TXOP duration value=1,024 μsis signaled in the same way as option 2-2 of Table 20 (e.g., 6-bitduration, small unit=16 μs and large unit=512 μs), the TXOP durationfield value (TXOP index) is set to 33.

TXOP Termination/Truncation Method

According to the above-described embodiment, the TXOP duration field ofHE-SIG A can set a TXOP duration on the basis of a relatively largegranularity. For example, the duration field included in the MAC headercan be indicated based on a 1 μs granularity, whereas the TXOP durationfield of HE-SIG A can be set to indicate a TXOP duration value on thebasis of a granularity greater than the granularity of 1 μs.

When the TXOP duration is set by the TXOP duration field of HE-SIG A onthe basis of a relatively large granularity, the TXOP duration can beset to a time longer than the time actually used for frame transmission.Accordingly, other STAs may set incorrect NAVs on the basis of HE-SIG Aand thus cannot use channels for a specific time, and channel efficiencymay be deteriorated.

To solve such problems, information for early termination of TXOP may betransmitted. For example, when a TXOP holder/responder transmits thelast frame (e.g., ACK, Block ACK, Multi-STA BA) during a TXOP period,the TXOP holder/responder may include information indicating earlytermination of TXOP in the last frame and transmit the last frame. EarlyTXOP termination may be represented as TXOP truncation or simply as(early) termination/truncation. A description will be given of TXOPtermination methods.

(1) Method Using CF-END Frame

A TXOP holder/responder can transmit the last frame during a TXOP periodand then terminate TXOP by transmitting a CF-END frame.

(2) Method Using Early Termination Indicator

According to an embodiment, an STA can include an early terminationindicator in part of a frame (e.g., a common part of HE-SIG A and HE-SIGB, etc.) and transmit the frame. For example, the STA can indicate earlyTXOP termination by setting early termination indicator=1. TXOP can beterminated immediately after the frame including early terminationindicator=1. The early termination indicator may be combined with theduration field when used. For example, early termination indicator=1 canindicate that TXOP is terminated at a time indicated by the durationfield. When Duration=0, TXOP can be terminated after the correspondingframe. When the duration field has a value greater than 0, TXOP can beterminated at a time indicated by the duration field.

(i) The MD (more data) field or ESOP field may be reused as the earlyTXOP termination indicator.

(ii) In the case of a DL frame, the early termination indicator can betransmitted in the last frame of a set TXOP. For example, a TXOPduration is updated and transmitted along with the early terminationindicator in the last frame. The TXOP duration is set to be less than aprevious TXOP duration and TXOP termination can be indicated through theearly termination indicator.

(iii) When TXOP information update is needed, an STA (e.g., a TXOPholder/responder) sets a TXOP updated when a frame is transmitted andtransmits the frame. In the frame in which the TXOP is updated, theearly termination indicator is used as a TXOP update indicator. Forexample, the early termination indicator can be set to 1 and transmittedwhenever the TXOP is updated. Upon reception of a frame in which theearly termination indicator is set to 1, an STA (e.g., a third partySTA) updates the TXOP of the corresponding STA (e.g., NAV update).

(iv) In the case of single frame (e.g., PPDU) transmission, the TXOPduration can be set to the size of ACK/BA. In the case of multi-frametransmission, the TXOP duration is set for multi-frame and ACK/BAtransmission.

(v) UL MU transmission: If a trigger frame is transmitted in a non-HTPPDU (e.g., 11a format), content of the trigger frame indicates acorrect TXOP duration and thus even a legacy STA (e.g., STA that doesnot support flax) can correctly set the TXOP duration (e.g., NAVsetting/update). In a UL MU frame, a TXOP duration corresponding to atransmission duration of an ACK/BA frame is indicated, and thus there isno problem in NAV setting/update.

However, when 11ax format is used and a TXOP duration set in HE-SIG Adiffers from TXOP duration information included in frame content (TXOPduration of the MAC header), a problem is generated. For example, someSTAs (e.g., third party) may read only HE-SIG A and other STAs (e.g.,third party) may read both the HE-SIG A and frame content.

STAs that have read both HE-SIG A and frame content set TXOP throughduration information of the frame content (e.g., MAC header). Forexample, the STAs that have read both HE-SIG A and frame content storethe duration information included in HE-SIG A. Upon read of the durationof the MAC header (or duration of the content), the STAs determine afinal TXOP duration on the basis of the duration of the MAC header (orduration of the content) instead of the duration of HE-SIG A to updateNAVs.

STAs that read only HE-SIG A update NAVs on the basis of the TXOPduration included in HE-SIG A. In this case, a problem that a TXOPduration longer than the actual TXOP duration of the MAC header is setmay be generated. For example, when an ACK/BA/M-BA frame in response toa UL MU frame is transmitted, the STAs update TXOP through TXOP durationinformation included in HE-SIG A/B or the MAC header (e.g., NAV update)and can terminate TXOP at a corresponding time when the earlytermination indicator (or TXOP update indicator) is set to 1.

(vi) TXOP termination may be performed on the basis of a BSS color. Forexample, an STA (e.g., third party) may be configured to terminate TXOPonly when TXOP termination is indicated through a frame corresponding toa BSS color thereof. The STA (e.g., third party) checks a BSS colorincluded in a frame. If the BSS color indicates other BSSs, the STA(e.g., third party) does not terminate TOXP even when the frameindicates TXOP termination. Accordingly, the STA (e.g., third party) canterminate/truncate TXOP only when a frame of the BSS thereof indicatesTXOP termination (e.g., explicit indication or implicit indication inwhich duration is set to 0). However, loss of access opportunity of theSTA for other BSSs may occur.

(vi) According to an embodiment, when an STA (e.g., a TXOPholder/responder) transmits 11ax frames within TXOP, the STA cannecessarily include TXOP termination/truncation information in the lastframe and transmit the last frame. In the case of 11a frames, correctTXOP can be set because TXOP is set through the duration of the MACheader. In an embodiment, the TXOP duration of HE-SIG may be overwrittenby the duration of the MAC header.

(3) Method Using Duration Field Value of Last Frame

According to an embodiment, an STA (e.g., a TXOP holder/responder) mayindicate early termination/truncation of TXOP by setting the durationfield value of the last frame to a specific value (e.g., setting theduration field value to 0 or setting all bits to 1) instead of using anexplicit TXOP termination indicator. Accordingly, upon reception of aframe indicating Duration=specific value (e.g., 0), an STA (e.g., thirdparty) can determine that the TXOP duration has beenterminated/truncated after the frame. This can be understood as afunction similar to the CF-END frame.

(4) NAV Management Method

According to existing NAV setting/update methods, NAV update isperformed only when a TXOP duration value of a received frame exceeds aNAV value currently set to an STA (e.g., third party). For early TXOPtermination, NAV update needs to be performed even when the TXOPduration value of the received frame is less than the NAV valuecurrently set to an STA. According to an embodiment, the STA may updatethe NAV with a TXOP duration less than the NAV value currently setthereto on the basis of the aforementioned TXOPtruncation/termination/update indicator. However, NAV update with a TXOPduration less than the currently set NAV value may be set to beperformed only on the basis of a TXOP termination/update indicatorincluded in myBSS frame.

The STA may set and maintain a NAV per BSS color. When the STA sets aNAV per BSS color, the STA can truncate the TXOP of the NAVcorresponding to a BSS color indicated by a frame indicating TXOPtruncation upon reception of the frame.

However, to reduce complexity of NAV setting and management, the STA mayset and maintain two types of NAVs, i.e., myBSS NAV and other BSS NAV(e.g., BSS other than myBSS or a frame that does not indicate myBSS).The term “myBSS” may be referred to as an intra-BSS NAV.

An operation for TXOP power reduction may be defined. For example,feasibility of NAV update is indicated, an STA (e.g., third party)maintains a wake-up state. If no NAV update is indicated, the STA canswitch to a power saving (PS) mode. To this end, an STA (e.g., a TXOPholder/responder) that sets a TXOP may include information about whetherNAV update will be performed in a frame and transmit the frame. The STA(e.g., third party) may switch to the PS mode only when a received frameis myBSS frame and indicates switching to the PS mode. (e.g., indicatesno NAV update). The STA (e.g., TXOP holder/responder) may not instructthe STA (e.g., third party) to switch to the PS mode when indicatingTXOP/NAV update through frame transmission and may instruct the STA(e.g., third party) to switch to the PS mode only when there is no NAVupdate.

FIG. 26 illustrates a frame transmission (e.g., TXOP management) and NAVmanagement (e.g., frame reception) method according to an embodiment ofthe present invention. Description of redundant parts in the abovedescription and the present embodiment will be omitted. It is assumedthat STA 1 and STA 3 are TXOP holder/responder STAs and STA 2 is a thirdparty STA. STA 1, STA 2 and STA 3 may be AP or non-AP STAs.

Referring to FIG. 26, STA 1 sets a first duration field (e.g., TXOPduration field) included in an HE-SIG A field. The first duration fieldmay be set to indicate a TXOP (transmission opportunity value) using asmaller number of bits than that of a second duration field (e.g., MACduration field) included in a MAC header. In addition, a granularity ofa time unit used for indicating a TXOP value in the first duration fieldmay be set to differ from a granularity (e.g., 1 μs) of a time unit usedin the second duration field of the MAC header. The second durationfield may be set to 15 bits.

For example, the granularity used in the first duration field may be setto an integer multiple of the granularity used in the second durationfield. Furthermore, the granularity used in the first duration field mayvary depending on a TXOP value to be indicated through the firstduration field.

The first duration field may include at least one bit (e.g., MSB)indicating a granularity determined according to a TXOP value. Theremaining bits of the first duration field may indicate the number oftime units included in a TXOP value based on the granularity indicatedby the at least one bit.

Specifically, the first duration field may be set to 5, 6 or 7 bits andthe MSB (most significant bit) of the first duration field may be usedto indicate a granularity. For example, the first duration field can beset to 5 bits and the granularity indicated by the MSB can be one of 32μs and 512 μs. As another example, the first duration field can be setto 6 bits and the granularity indicated by the MSB can be one of 16 μsand 256 μs. In another example, the first duration field can be set to 7bits and the granularity indicated by the MSB can be one of 8 μs and 128μs.

Both the TXOP value indicated by the first duration field and the TXOPvalue indicated by the second duration field (e.g., MAC duration) may beset for transmission of the same frame. However, the TXOP valueindicated by the first duration field can be calculated on the basis ofthe TXOP value indicated by the second duration field. The TXOP valueindicated by the first duration field may be determined to be greaterthan or equals to the TXOP value indicated by the second duration field.

STA 1 transmits frame 1 including an HE-SIG field and a MAC header(S2601).

It is assumed that STA 3 is designated as a receiver of frame 1 forconvenience of description. For example, it is assumed that a receiveraddress field of frame 1 transmitted by STA 1 is set to the address ofSTA 3 (e.g., the MAC address or AID of STA 3). Accordingly, STA 1/STA 3are TXOP holders/responders and STA 2 is a third party STA.

STA 2 receives (or detects) frame 1 transmitted form STA 1 to STA 3(S2615).

STA 2 may perform NAV management on the basis of one of the firstduration field included in the HE-SIG A field and the second durationfield included in the MAC header (S2615). NAV management may refer tosetting, update or resetting a time period at which channel access isrestricted in order to protect the TXOP of the transmitter of frame 1(e.g., STA 1) or the receiver of frame 1 (e.g., STA 3) when STA 2 is notdesignated as a receiver of frame 1. It is assumed that STA 2 does nothave a currently set NAV value (e.g., NAV=0) for convenience.Accordingly, STA 2 sets a NAV on the basis of frame 1.

An STA performing NAV management can perform NAV management on the basisof the second duration field (e.g., MAC header) upon successful MACheader decoding and perform NAV management on the basis of the firstduration field (e.g., HE-SIG A) upon MAC header decoding failure. In thepresent embodiment, it is assumed that second STA 2 sets a NAV on thebasis of the first duration field (e.g., HE-SIG A) for convenience.

Upon reception of frame 1, STA 3 transmits frame 2 including an HE-SIG Aand a MAC header to STA 1 (S2620). STA 2 can detect (or receive) frame 2and update or reset a NAV on the basis of frame 2 (S2625).

FIG. 27 is an explanatory diagram of apparatuses for implementing theaforementioned method.

A wireless device 800 and a wireless device 850 in FIG. 27 maycorrespond to the aforementioned STA/AP 1 and STA/AP 2, respectively.

The STA 800 may include a processor 810, a memory 820, and a transceiver830 and the AP 850 may include a processor 860, a memory 870, and atransceiver 860. The transceivers 830 and 880 may transmit/receive awireless signal and may be implemented in a physical layer of IEEE802.11/3GPP. The processors 810 and 860 are implemented in a physicallayer and/or a MAC layer and are connected to the transceivers 830 and880. The processors 810 and 860 may perform the above-described UL MUscheduling procedure.

The processors 810 and 860 and/or the transceivers 830 and 880 mayinclude an Application-Specific Integrated Circuit (ASIC), a chipset, alogical circuit, and/or a data processor. The memories 820 and 870 mayinclude a Read-Only Memory (ROM), a Random Access Memory (RAM), a flashmemory, a memory card, a storage medium, and/or a storage unit. If anexample is performed by software, the above-described method may beexecuted in the form of a module (e.g., a process or a function)performing the above-described function. The module may be stored in thememories 820 and 870 and executed by the processors 810 and 860. Thememories 820 and 870 may be located at the interior or exterior of theprocessors 810 and 860 and may be connected to the processors 810 and860 via known means.

The detailed description of the preferred examples of the presentinvention has been given to enable those skilled in the art to implementand practice the invention. Although the invention has been describedwith reference to the preferred examples, those skilled in the art willappreciate that various modifications and variations can be made in thepresent invention without departing from the spirit or scope of theinvention described in the appended claims. Accordingly, the inventionshould not be limited to the specific examples described herein, butshould be accorded the broadest scope consistent with the principles andnovel features disclosed herein.

INDUSTRIAL APPLICABILITY

The present invention has been described on the assumption that thepresent invention is applied to a wireless LAN system supporting HEPPDUs. However, the present invention is not limited thereto and can beapplied to various wireless communication systems including IEEE 802.11.

The invention claimed is:
 1. A method for obtaining transmissionopportunity (TXOP) information by a station (STA) in a wireless localarea network (WLAN), the method comprising: detecting a high efficiency(HE) physical protocol data unit (PPDU) including a HE-signal A (SIG A)field; and obtaining a TXOP duration value based on a first durationsubfield of the HE-SIG A field of the HE PPDU, wherein the firstduration subfield includes a plurality of bits, wherein a front part ofthe plurality of bits in the first duration subfield indicates whether agranularity for the TXOP duration value is ‘A’ micro-second (us) or ‘B’us, where ‘A’ and ‘B’ are predetermined integers greater than 1, andwherein a remaining part of the plurality of bits in the first durationsubfield includes scaled duration information that is scaled by thegranularity for the TXOP duration value.
 2. The method of claim 1,wherein the STA identifies whether the granularity for the TXOP durationvalue is ‘A’ us or ‘B’ us based on a front part of the plurality of bitsin the first duration subfield, wherein the STA identifies the scaledduration information based on the remaining part of the plurality ofbits in the first duration subfield and wherein the STA obtains the TXOPduration value based on a combination of the identified granularity andthe identified scaled duration information.
 3. The method of claim 1,wherein the STA is configured with a look-up table including a pluralityof TXOP duration values which correspond to combinations ofgranularities and scaled durations, and wherein the STA selects the TXOPduration value from among the plurality of TXOP duration values of thelook-up table based on the first duration subfield.
 4. The method ofclaim 1, wherein the HE PPDU includes a medium access control (MAC)header, and the MAC header includes a second duration subfield, whereina granularity used for the second duration subfield is 1 us, and whereina total number of the plurality of bits in the first duration subfieldis smaller than that of the second duration subfield.
 5. The method ofclaim 4, wherein the STA sets a network allocation vector (NAV) based onthe TXOP duration value obtained from the first duration subfield ofHE-SIG A field, when the STA is not able to obtain duration informationfrom the second duration subfield of the MAC header, and wherein the STAsets the NAV based on the duration information from the second durationsubfield of the MAC header, when the STA has obtained the durationinformation from the second duration subfield, even if the TXOP durationvalue was obtained from the first duration subfield of HE-SIG A field.6. The method of claim 1, wherein the front part is an initial 1-bit ofthe plurality of bits, wherein if the initial bit of the first durationsubfield is 0, the TXOP duration value corresponds to ‘A*the scaledduration information’ us, and wherein if the initial bit of the firstduration subfield is 1, the TXOP duration value corresponds to‘512+B*the scaled duration information’ us.
 7. The method of claim 6,wherein ‘A’ is 8, ‘B’ is 128, and the first duration subfield is 7-bit.8. The method of claim 1, further comprising: setting a networkallocation vector (NAV) based on the HE PPDU when the STA is a thirdparty STA for the HE PPDU, wherein the setting of the NAV corresponds toone of an initial setting, updating or resetting the NAV.
 9. The methodof claim 8, wherein the STA performs a basic service set (BSS)identifying operation for the HE PPDU before setting the NAV, wherein ifthe HE PPDU is identified as an intra-BSS PPDU, the STA selects a firstNAV for the setting of the NAV from 2 NAVs maintained by the STA, andwherein if the HE PPDU is not identified as the intra-BSS PPDU, the STAselects a second NAV for the setting of the NAV from the 2 NAVsmaintained by the STA.
 10. A non-transitory computer readable mediumrecorded thereon a program for executing the method of claim
 1. 11. Astation (STA) comprising: a transceiver; and a processor to detect,through the transceiver, a high efficiency (HE) physical protocol dataunit (PPDU) including a HE-signal A (SIG A) field and to obtain atransmission opportunity (TXOP) duration value from a first durationsubfield of the HE-SIG A field of the HE PPDU, wherein a front part ofthe plurality of bits in the first duration subfield indicates whether agranularity for the TXOP duration value is ‘A’ micro-second (us) or ‘B’us, where ‘A’ and ‘B’ are predetermined integers greater than 1, andwherein a remaining part of the plurality of bits in the first durationsubfield includes scaled duration information that is scaled by thegranularity for the TXOP duration value.
 12. The STA of claim 11,wherein the processor identifies whether the granularity for the TXOPduration value is ‘A’ us or ‘B’ us based on a front part of theplurality of bits in the first duration subfield, wherein the processoridentifies the scaled duration information based on the remaining partof the plurality of bits in the first duration subfield and wherein theprocessor obtains the TXOP duration value based on a combination of theidentified granularity and the identified scaled duration information.13. The STA of claim 11, further comprising: a memory to store a look-uptable including a plurality of TXOP duration values which correspond tocombinations of granularities and scaled durations, and wherein theprocessor selects the TXOP duration value from among the plurality ofTXOP duration values of the look-up table based on the first durationsubfield.
 14. The STA of claim 11, wherein the HE PPDU includes a mediumaccess control (MAC) header, and the MAC header includes a secondduration subfield, wherein a granularity used for the second durationsubfield is 1 us, and wherein a total number of the plurality of bits inthe first duration subfield is smaller than that of the second durationsubfield.
 15. The STA of claim 14, wherein the processor sets a networkallocation vector (NAV) based on the TXOP duration value obtained fromthe first duration subfield of HE-SIG A field, when the processor is notable to obtain duration information from the second duration subfield ofthe MAC header, and wherein the processor sets the NAV based on theduration information from the second duration subfield of the MACheader, when the processor has obtained the duration information fromthe second duration subfield, even if the TXOP duration value wasobtained from the first duration subfield of HE-SIG A field.
 16. The STAof claim 11, wherein the front part is an initial 1-bit of the pluralityof bits, wherein if the initial bit of the first duration subfield is 0,the TXOP duration value corresponds to ‘A*the scaled durationinformation’ us, and wherein if the initial bit of the first durationsubfield is 1, the TXOP duration value corresponds to ‘512+B*the scaledduration information’ us.
 17. The STA of claim 16, wherein ‘A’ is 8, ‘B’is 128, and the first duration subfield is 7-bit.
 18. The STA of claim11, wherein the processor sets a network allocation vector (NAV) basedon the HE PPDU when the STA is a third party STA for the HE PPDU,wherein the setting of the NAV corresponds to one of an initial setting,updating or resetting the NAV.
 19. The STA of claim 18, wherein theprocessor performs a basic service set (BSS) identifying operation forthe HE PPDU before setting the NAV, wherein if the HE PPDU is identifiedas an intra-BSS PPDU, the processor selects a first NAV for the settingof the NAV from 2 NAVs maintained by the processor, and wherein if theHE PPDU is not identified as the intra-BSS PPDU, the processor selects asecond NAV for the setting of the NAV from the 2 NAVs maintained by theprocessor.